Patent Searching Made Easy
How to Do Patent Searches on the Internet & in the Library
April 2013, 6th Edition
Inventor? Find out if you're the first to file a patent, online and in the library.
In the past, if you wanted to assess the novelty of an idea, you had to wade through the patent database at the U.S. Patent and Trademark Office (PTO) in Virginia -- or hire a lawyer to do a patent search for $500 and up. The cost and inconvenience of these searches often meant that good ideas were left to rot on the vine.
In Patent Searching Made Easy, find the plain-English information you need to:
- verify the patent status of an idea
- search Patent and Trademark Depository Libraries
- use online patent search services
A physicist, engineer, and patent searching expert, author David Hitchcock gives you the vocabulary, instructions and strategies you need to search for a patent quickly and easily. He explains how the PTO classifies different types of inventions, so that you can assign your idea to the right class, compare it to related ideas and then determine if it's novel enough to qualify for a patent.
Patent Searching Made Easy shows you how do patent searches yourself, on the Internet, at little or no cost. Plus, you'll learn how to:
- prepare for online searches with the right hardware, software and computer skills
- access online patent searching resources
- narrow online searches with keywords and Boolean logic
- perform database searches at Patent and Trademark Depository Libraries (PTDLs)
- do microfiche searches at PTDLs
- search fee-based patent databases on the Internet
- search international patent offices
- do advanced searches at the PTO and PTDL
Written for both inventors and business owners interested in expanding their product line through the license, distribution or manufacture of other people's ideas, Patent Searching Made Easy is the easiest way for you to determine the answer to that all-important question, "Am I the first?"
“A word of caution before trying a search at the U.S. Patent and Trademark Office website—first read Patent Searching Made Easy.”
- Popular Mechanics
“With patent searches costing anywhere from $300 to $700, this book should be a valuable aid to most inventors.”
- Entrepreneur Magazine
“This book thoroughly explains how to search for previously issued U.S. patents…. The explanations are helpful for both computer expert and ‘newbie.”
Class Finder Tool
David Hitchcock is a physicist and engineer who has worked as a computer consultant on such diverse projects as the MX missile, the Milstar satellite program and advanced capability torpedoes. He has focused on patent searching and new technology for a number of years. Hitchcock is the inventor of the Home First Shield, a device to protect homes from wildfires.
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TABLE OF CONTENTS
Your Legal Companion 1
Part I: The Basics
Patents and Patent Searching 7
What Is a Patent and What Does It Do for Me? 8
What Is Prior Art? 9
What Is the PTO Patent Database? 10
Patent Searching and Patent Eligibility 12
The Easy Part: Statutory Subject Matter and Usefulness 12
Novelty: Is Your Invention New? 13
Nonobviousness: Is Your Invention Obvious? 17
The Patent Document 20
Understanding Patent Databases 29
How Information Is Entered Into the Database 30
Understanding Keyword Searching 31
What You Need to Search Patent Databases 42
Part II: Simple and Advanced Searching
Patent Searching at the PTO Website 47
Simple Keyword Searches at the PTO's Website 48
Searching the Manual of Classification 60
Search by Patent Number 71
Working With Patent Images 73
An Effective Strategy for Basic Patent Searches 80
Advanced Search Techniques 87
How to Accomplish an Advanced Patent Search at the PTO Website 88
Searching Published Patent Applications at the PTO's Website 106
Patent Searching at the EPO Website 113
Patent Searching at the EPO 114
Other Patent Search Websites 131
Google Patent Search 132
Fee-Based Patent-Searching Websites 139
Additional Sources of Prior Art 141
The Thomas Register 149
Government Websites 152
Usenet Newsgroups 156
Trade Magazines, Books, and Stores 160
Part III: Resources
Hitting the Books 163
Index to the U.S. Patent Classification System 164
Manual of Classification 169
Classification Definitions 171
Using CASSIS 181
The CASSIS DVD Database 182
Welcome to CASSIS2 182
Additional CASSIS Features 193
EAST Meets WEST 195
Web-based Examiner Search Tool (WEST) 197
Examiner Automated Search Tool (EAST) 203
Additional EAST/ WEST Features 208
Reference Collection of U.S. Patents Available for Public Use in Patent and Trademark Resource Centers 217
In Chapter 1, we discuss what a patent is, the basic principles of searching, and the relationship between your search and the patentability of your invention. In Chapter 2, we introduce you to word-based patent searches and the resources you will need for patent searching.
Patents and Patent Searching
What Is a Patent and What Does It Do for Me?.............................. 8
What Is Prior Art?............................................................................ 9
What Is the PTO Patent Database?.............................................. 10
Patent Searching and Patent Eligibility.......................................... 12
The Easy Part: Statutory Subject Matter and Usefulness............ 12
Novelty: Is Your Invention New?................................................... 13
Physical Differences................................................................. 14
Combinatorial Differences......................................................... 16
New Uses.................................................................................. 17
A Systematic Approach to Novelty........................................... 17
Nonobviousness: Is Your Invention Obvious?............................... 17
The Patent Document................................................................... 20
In order to receive a patent, your invention must be both new (novel) and surprising in light of prior developments (not obvious). Both of these standards are judged not only against all previously issued patents, but also against all previous developments in the same field, whether or not they were ever patented. For instance, the grooves in an automobile steering wheel were deemed to be a nonpatentable invention because of the traditional use of grooves in sword handles.
So, the key to assessing the patentability of your new idea is understanding what previous developments—known in the trade as prior art—the U.S. Patent and Trademark Office (PTO) will consider when deciding whether to issue a patent on your idea. In order to proceed, you must first understand the nature of patents and prior art.
What Is a Patent and What Does It Do for Me?
A patent is a right, granted by the government, to a person or legal entity (partnership or corporation). A patent gives its holder the right to exclude others from making, using, or selling the invention “claimed” in the patent deed for 20 years from the date of filing (for patents issued before June 8, 1995, 17 years from the date the patent was issued by the PTO). Once the patent expires, the invention covered by the patent enters the public domain and can be used by anyone. The scope of a U.S. patent is limited to the borders of the United States and its territories.
The right of exclusion given to a patent owner is referred to as an offensive legal right—meaning that the owner must go on the offensive to protect patent rights. That may mean filing a lawsuit in federal court against an infringer (anyone who violates the right of exclusion). Because the right of exclusion is not a defensive legal right, the patent owner can’t rely on law enforcement agencies to automatically prosecute someone who infringes (copies) his or her patented invention.
In the sense that a patent gives the patent holder the right to sue anyone who tries to develop, use, or manufacture the invention covered by the patent, the patent can be a valuable commodity. It can be sold outright or licensed in exchange for a royalty. Additionally, the patent owner may choose to manufacture and distribute the invention, thereby keeping all the proceeds for him- or herself.
What Is Prior Art?
After March 16, 2013, (when the America Invents Act takes full effect) “prior art” consists of any knowledge that was made publicly available prior to the filing date of your patent application (unless such knowledge came from you (the inventor) and was not disclosed over a year before the filing date). Also, under these new first-to-file rules (FTF), prior art also includes a U.S. patent application of another inventor, that is based on an earlier foreign or domestic application and is entitled to the earlier application’s filing (priority) date; the earlier application’s filing date is the prior-art date. Finally, under FTF, prior art includes prior use and on-sale activities of the invention, even if done in another country. (Prior to March 16, 2013, U.S. patents were granted to the first-to-invent.) As long as you can demonstrate that your innovation differs physically in some way from all prior developments and concepts, the prior art will not affect your ability to patent your invention.
On the other hand, if prior art is uncovered by a patent examiner that demonstrates somebody already came up with the same idea as you—that is, all of the significant elements in your innovation were embodied in an existing innovation—you will be unable to obtain a patent and you’ll lose the thousands of dollars spent on application or attorney fees. (This is referred to as “prior art anticipation.”) In order to avoid this fate, and to help you improve your invention’s chance of acquiring patent rights, we strongly recommend searching for prior art.
After March 16, 2013, the definition of prior art—everything available before the date of filing the application—can also be summarized as including:
any published writing (including any patent) that was publicly available before you filed your patent application
any U.S. patent that has a filing date earlier than your date of filing
any relevant invention or development (whether described in writing or not) existing prior to the date your application was filed, or
any public or commercial use, sale, or knowledge of the invention prior to your application filing date unless such disclosure came from you (the inventor) and was not disclosed over a year before the filing date.
For your purposes, it’s best to spread as wide a prior art search net as possible, and to spread this net, you will eventually need to go beyond the database of patents at the PTO and examine all written references and all real-life items that may embody your idea.
However, for now, we’ll begin your research with a basic and inexpensive search of the U.S. patent database. That’s because if someone has thought of your idea before and deemed it valuable, chances are good that the idea will show up in one or more patents. Keep in mind, however, that pending patent applications (patent applications that have already been submitted, but for which no patent has yet been issued) cannot be searched until 18 months after the application was filed. (We explain more about this later in this chapter.)
What Is the PTO Patent Database?
Much of your patent searching will be accomplished by searching the U.S. patent database which contains all the patents issued by the PTO. These patents are stored in patent file folders at the PTO in Virginia, and the PTO has created a computer database of patent images and text.
The traditional method of searching the patent database is to hire a search professional to travel to the PTO’s offices in Virginia and conduct the search there. While very effective, this process is also very expensive. Instead of starting with this approach, you can save yourself some money by performing a preliminary search. If your search reveals that your idea has already been described in one or more previous patents, you will have saved yourself the expense of hiring a search professional.
The PTO provides an online database where you simply type in words which describe your invention—called keywords—to search for patents as far back as 1976 that contain those same words. Pre-1976 patents can also be searched on a much more limited basis. By the way, the search engine company Google goes one step further: The company has scanned all U.S. patents in the PTO database and permits you to search even pre-1976 patents using keyword searching techniques at its Google Patent Search website.
In addition, a great resource for patent searching is a network of special libraries called Patent and Trademark Research Centers (PTRCs—see Appendix A for a list). At a PTRC you can perform computer searches of the PTO’s electronic database.
As you search the PTO patent database, you also will learn how to think about your ideas in the same way that the examiners at the PTO would think of them, were you to apply for a patent. This knowledge will enable you to search for ideas that are not only the same as yours, but similar to yours. This process will allow you to determine not only if your invention is the first, but also whether it is the best. And if it is not, the search may inspire you to refine your idea in ways that will qualify it for a patent.
As you will also see, searching the PTO patent database is a great way to become familiar with patent terminology. This will come in handy during all aspects of the patent search as well as the patent application process itself. In particular, when dealing directly with the patent examiner who is reviewing your application, it helps if you are both speaking the same language.
Patent Searching and Patent Eligibility
In order to search properly, you will need a short primer on what qualifies for a patent. This primer will help you decide—once you complete your search—whether you should go through the effort and expense of filing a patent application. Keep in mind that this is a brief, simple explanation of patentability. Before filing a patent (and for a more thorough analysis of patentability), we recommend you peruse a more detailed analysis. A good way to start would be by reviewing Patent It Yourself by attorney David Pressman (Nolo).
In order to get a utility patent (as opposed to a design patent), your patent application has to satisfy four legal criteria:
It must fit into an established statutory class.
It must be useful.
It must be novel—that is, have some physical difference from any similar inventions in the past.
It must be unobvious (also referred to as nonobvious) to someone who is skilled in the appropriate field.
The Easy Part: Statutory Subject Matter and Usefulness
Two patent standards, statutory subject matter and usefulness, are easy to achieve. The first requirement—statutory subject matter—means that your invention must be a process, a machine, an article of manufacture, a composition of matter, or a new use invention. It’s not necessary to decide which applies to your invention—and many inventions overlap—as long as your invention is covered by at least one of them.
The second requirement is utility—that is, is your invention useful? If your invention is operable (if it functions), it will satisfy this requirement. Perpetual motion machines or other devices that violate an established law of physics are examples of inventions that fail this requirement.
Statutory Subject Matter
A process is the performance of a series of operations on something—for example, electroplating or a process for scanning election results.
A machine is a device consisting of a series of fixed or moving parts that direct mechanical energy toward a specific task—for example, an automobile engine, turbine, or drill press.
These are relatively simple inventions with few or no moving parts—for example, a screwdriver, rake, pencil, or mirror.
Compositions are a unique arrangement of items—for example, glue and plastics.
A new use is a new way of using an invention that fits in one of the first four statutory classes—for example, using birth control pills to limit acne or a method of converting a video game controller into a laser pointer.
Novelty: Is Your Invention New?
At the heart of patent searching—and the goal of this book—is demonstrating that your invention is novel. Demonstrating novelty means that you must be able to prove that no single prior art item describes all of your invention’s significant elements. If your search uncovers inventions that perform the same or similar functions, you will need to somehow distinguish your innovation from these previous inventions (whether patented or not). You can distinguish your invention based on:
combinatorial differences, or
An example of a physical difference between your invention and a previously patented product would be the elimination, replacement, or functional modification of a component of the previous device.
Consider the invention shown in Figure 1.1. The figure shows a side and front view of a fire safety glass window. It consists of four elements.
Element #1. Left-hand side glass segment
Element #2. A thermal conduction film
Element #3. Right-hand side glass segment
Element #4. Heat-conducting metal frame
In order for this invention to work, a heat-conducting film is sandwiched between two glass plates. When a sharp temperature rise occurs on either side of the glass, the film conducts the heat away from the window to the metal frame.
Now consider the invention shown in Figure 1.2. This figure is also a side and front view of a safety glass window. In this case there are three elements:
Element #1. Thermal-conducting wire mesh
Element #2. Glass segment enclosing the wire mesh
Element #3. Heat-conducting metal frame
The wire mesh conducts heat away from the glass and into the frame, just as the first invention does. But now, instead of two separate glass elements, there is only one continuous glass element (in which the wire grid is embedded).
As an example of a replacement physical difference, consider the centrifugal water pump shown in Figure 1.3. Centrifugal pumps use impellers to impart energy to the water. For the sake of simplicity, we will identify four essential elements.
Element #1. Pump intake
Element #2. Metal impeller
Element #3. Pump casting
Element #4. Pump discharge
Suppose the impeller is made out of metal and you design a new impeller for this pump that performs as well, but with a different design and made out of plastic. This replacement physical difference would satisfy the novelty requirement.
As an example of a functional modification, suppose our water pump vibrates at high pressure (high impeller speed) due to water turbulence. You redesign the blades of the impeller by increasing their pitch. This solves the turbulence problem and allows the pump to operate at high pressure. You have therefore made a functional modification.
A new combination of two different inventions can also be used to satisfy the novelty requirement—for example, the combination of a hot air balloon (an old invention) and a new high-strength lightweight fabric. The lightweight fabric replaces the older balloon material, making the balloon lighter. This provides more lift and lets the balloon carry more cargo.
A new use of an old invention can also satisfy the novelty requirement. As an example, suppose during World War II, a sonar engineer developed a sonar receiver that detects the sound of a ship’s propeller. Several years later, an independent inventor designs a pool alarm that uses the same technology to sound an alarm if a child accidentally falls into a pool. Even though the pool alarm uses the same electronics, it would pass the new use test.
A Systematic Approach to Novelty
A four-step systematic approach can be made for assessing novelty.
Analyze your invention for elements.
Analyze each prior art reference for its elements.
Compare the elements of each prior art reference to the elements of your invention.
If no one prior art reference contains all of the same elements used in the same way and for the same purpose as your invention, then it is novel.
Nonobviousness: Is Your Invention Obvious?
This is the most challenging patent requirement to fulfill. Essentially, it means that if a skilled worker who is thoroughly familiar with developments in the area of your invention would consider the idea obvious, you would fail this test. As an example, consider the balloon made with new lightweight fabric from the previous page. While this invention qualifies as novel, it would be obvious to a person skilled in the art of balloon making to try using new lightweight materials.
The Patent Application Process
When you submit your completed patent application and filing fee to the PTO, you will be assigned a filing date. After a waiting period (commonly 6 to 18 months), a patent examiner will review the application. It is extremely rare that an application will be allowed as is. More often than not, the patent examiner will object to one or more of your claims or require changes to your patent drawings or specification. This result is what is known as an Office Action (or OA). The OA is an official communication from the PTO, outlining the objections to your original patent application. You then have the choice of either modifying the application or convincing the examiner that your application is correct as written—that is, the examiner is in error.
After you successfully respond to the OA, the patent examiner will allow your application and you will have to pay an issue fee. After a few more months’ delay, your patent will finally issue. The entire process, from initial patent application submission to issued patent, usually takes from 16 to 36 months.
For more information about how to complete and file a patent application, see Patent It Yourself, by attorney David Pressman (Nolo).
Types of Patents
There are three types of patents: utility patents, design patents, and plant patents. In this book, we focus on searching for utility patents, the most common of the three, and, unless otherwise indicated, when we speak of a patent, we are referring to a utility patent. (Note: The rules for searching all types of patents are similar.)
A utility patent covers the functional aspects of an invention. As an example, assume that the hammer hasn’t been invented. Ivan Inventor conceives of the hammer as an invention after he accidentally smashes his thumb with a rock he was using to pound a square peg into a round hole. If Ivan applies for a patent and his patent application describes his hammer invention in general enough terms, the patent would cover all variations of the hammer as a utilitarian device. It would cover common household hammers, sledge hammers, rubber hammers, and the like. Perhaps even hydraulic hammers could be covered.
A design patent only protects the appearance of an invention. In our example, Ivan might apply for a design patent for a hammer with a horsehead etched into the shaft of the hammer. Removal of the horsehead would not affect the usefulness or functioning of the hammer. It is easy to create a fairly similar design without infringing a design patent (referred to as designing around the patent). A competitor could design a hammer with a slightly different horsehead (longer mane or bigger eyes), and the new hammer design most likely would not infringe on the original design patent.
Plant patents are for new types of plants. We don’t discuss them in this book.
Pending Patent Applications
One of the pitfalls of any patent search is that there is no way to search pending patent applications prior to 18 months from filing. Another inventor may have already filed a patent application on essentially the same invention as you. Because pending patent applications are kept confidential (prior to 18 months), if the patent hasn’t been issued yet you have no way of knowing about it. This is one of the occupational hazards involved in applying for a patent.
If you have a patent application pending and a patent is issued that covers the essential aspects of your idea, you may still be able to get a patent. How? You do it by proving that you filed your application earlier than the inventor listed in the opposing patent.
The Patent Document
Although we have explained some patent basics—for example, the PTO issues patents; patents provide certain affirmative rights; and a patent expires in 20 years—the idea of a patent is still an abstraction.
A patent manifests itself in its most “real” form in a document called a patent deed. This is the document issued by the PTO after your patent application has been approved and your patent rights have issued. The patent deed, commonly referred to as a patent or patent reference—is a collection of words and drawings that summarize how to make and use your invention. This document also defines the boundaries of your rights, known as the claims. The PTO’s patent database comprises hardcopy, microfiche, or electronic copies of these patent documents.
Every patent has several identifiable fields or sections. Understanding the different parts of the patent will be especially important when we cover computer searching. This is because we will conduct our search in certain subsections of the patent, and it helps to know what sort of information to expect to find there.
We have prepared a table showing the typical sections that appear in a patent, along with a brief description. The table introduces you to several terms commonly used in the patent world. These include:
Classification (class and subclass). These refer to the system used by the PTO to categorize each patent. Conceptually, the system is similar to an alphabetical library index file. For example, to search a library for a book about baseball, one would first go to the subject card index. In the file drawer for subjects beginning with the letter S, you would most likely find a sports section. under the sports section, you would go to the subsection for baseball. There you would find the titles of several books related to baseball. The PTO currently has more than 100,000 classes and subclasses.
Abstract. An abstract is a summary of the most important features of the invention covered by the patent. The abstract appears on the front page of the issued patent. Patent searchers consult the abstract to get a quick overview of the invention. This, in turn, helps them decide whether it is worthwhile to review the entire patent. The abstract is the searcher’s way to separate the wheat from the chaff. A typical abstract—from Pat. No. 5,712,618—is shown below.
An automatic signaling device for a vehicle which automatically initiates a method and apparatus for an automatic signaling device warning signal to pedestrians and to other vehicles in connection with lane changes and upon turns. The present invention is activated and deactivated automatically, providing significant safety advantages for all of those using the roads and highways.
15 Claims, 2 Drawing Figures
Background of the invention. This is a discussion of previous inventions that are related in some way to the current invention. These inventions are known as the prior art. The prior art may embody some of the same or similar elements as the current invention. For example, sprinkler systems and fireproof blankets are two vastly different products. However, they are both related by the fact that they are fire suppressant devices. So, if you invented a modern-day fire suppression device (for instance, one using nanotechnology—tiny microscopic machines—to deprive the fire of oxygen), both sprinkler systems and fireproof blankets would be considered prior art related to your invention.
The first two paragraphs from the background section of Pat. No. 5,712,618 are shown below. The first paragraph is a general summary of the background of the invention. The next paragraph begins the discussion of the advantages of the current invention over previously patented inventions.
The invention disclosed herein relates to preferred methods and apparatuses for an automatic signaling device which automatically activates a warning signal. The following patents form a background for the instant invention. None of the cited publications is believed to detract from the patentability of the claimed invention.
U.S. Pat. No. 3,771,096 issued to Walter on Nov. 6, 1973, discloses a lane changing signaling device for vehicles employing a rotary electrical connector joined to the steering wheel. The principal disadvantage of the device is that it fails to measure the angle of rotation of the steering wheel.
Detailed description of the preferred version of the invention. This is a detailed description of an actual, “nuts and bolts” version of the current invention (“embodiment” in patent terms). It is essentially the inventor’s best-guess (preferred embodiment) description of the product at the time the patent application is written. By reading the detailed description, a person who is familiar with similar products should be able to build and operate the current invention. It is important to note that the legal scope of the patent is not defined (the language of patents calls it “limited”) by the details of the description of the preferred embodiment. Rather, the scope of the patent is actually determined by the claims.
Patent Fields and Sections
This is the name or title of the invention.
This is the inventor’s identifying information—that is, name and address.
This is the number assigned by the PTO to the issued patent.
Patent Filing Date
This is the date that the patent application was filed with the PTO.
Patent Issue Date
This is the date that the patent was issued by the PTO.
These are the categories (the class and subclass) that the PTO uses to classify or sort the various types of inventions.
These are the patent numbers of previous patents referred to in the patent application, along with their classes and subclasses.
Usually one concise paragraph, the abstract summarizes the invention in layperson terms. The abstract appears on the front page of the patent and is the most frequently referenced section.
These are black-and-white drawings of the invention as seen from different perspectives.
Background of the Invention
This is a discussion of any previous inventions that were related to this invention (known as prior art).
Summary of the Invention
Here is a discussion of the invention that captures its essential functions and features.
Brief Description of Drawings
These are one-sentence descriptions of each patent drawing figure.
Detailed Description of the Preferred Version of the Invention
This is an in-depth discussion of the various aspects of the invention and in which painstaking references to the patent drawings are made.
This section defines the legal scope of the patent (in the way that a deed describes the boundaries of real estate).
Description of the preferred embodiment for a fire safety glass panel
Referring to FIG. 1, a fire-protection and safety glass panel according to a preferred embodiment of this invention comprises a first glass plate 10, a second glass plate 11 opposite to the first glass plate, and an intermediate resin layer between the first and second glass plates 10 and 11. At least one of the first and the second glass plates 10 and 11 is a heat-resistant glass plate. The intermediate resin layer comprises a polyethylene terephthalate film (namely, a PET film) 12 and first and second adhesive agent layers 13 and 14 and has a thickness which is not greater than 200 μm. The first adhesive agent layer 13 adheres the PET film 12 to the first glass plate 10.
The first paragraph of the detailed description of the preferred embodiment for Pat. No. 5,462,805 is shown above. Reading through the description, we see that specific numbered elements of FIG. 1 (from Pat. No. 5,462,805) are referenced. This figure is shown as Figure 1.4 above. Here we have a glass plate (Element 10), another glass plate (Element 11), an intermediate resin layer (Element 12), and first and second adhesive layers (Elements 13 and 14). By following along with the detailed description, and matching the numbered elements of the description with the labeled elements of the drawing, a person familiar with fire safety glass would be able to construct this invention.
Claims. Patent claims are a series of carefully worded statements that precisely describe and define the underlying invention. Patent claims operate in much the same way as real estate deeds—they precisely delimit the scope of the patent in the same way as the real estate deed describes the precise location of the property.
From the patent applicant’s viewpoint, the claims should be as broad as possible, thus covering many possible versions of the same basic invention. Broad claims make it difficult for someone to defeat the patent by making a minor change to the invention. On the other hand, if patent claims are too broad, there is always the possibility of someone finding a previous invention (prior art reference) that falls within the patent’s scope. This could make the patent susceptible to being ruled invalid if the patent holder ever finds it necessary to bring an infringement case.
The first claim from the fire safety glass patent (5,462,805) is shown below. While calling out the same elements of the invention as described in the preferred embodiment, the specific element references have been omitted. This is because the claim is meant to be general enough to include different designs based upon the same invention concept.
You will note that here, the glass plates are referred to as “low-expansion crystallized glass.” This is broad enough to include many types of glass that do not r