Edward Weston

Biography
One of the outstanding contributors to the development of the carbon filament, as well as the history of the incandescent lamp, was Edward Weston (May 9 1850 - Aug 20 1936). In addition, Weston made important contributions to incandescent lighting, arc lighting, electroplating, getters, dynamos and electrical instrument development. In this brief look, Weston's work with the tamadine filament is considered.

Development of the Tamidine Filament
In 1893 Weston classified the characteristics of carbon filaments as they depended on the material used as well as their subsequent treatments ("The Incandescent Lamp," Prof. Edward Weston's Remarks Before the St. Louis Convention of the National Electric Light Association⁴. It is of interest to review some of his comments. Five classes were given:

1) Carbons made from cellulose, the physical characteristics which have been modified by some process of manufacture which has, more or less, broken up the fibres or rearranged them and bound them together mechanically. Under this head I would include paper, cotton or linen thread or braid, and, as being closely allied, I would also include silk thread or braid.
"All such bodies have the fibres more or less broken up and interlaced in a more or less irregular fashion, but are not cemented together with cementing material, such as binds woody fibres together in their natural condition.
"Thus the fibres of paper are not parallel, but lie criss-cross in every direction, like the fibres of wool in a felt hat, and the points of contact are uncertain and very irregular.
"The fibres of cotton, linen or silk thread, are longer, yet not parallel, but are twisted in several strands, without any cementing or binding material to unite the strands firmly.

2) Carbons made from the previously named manufactured products, but which have been subsequently subjected to some chemical process, such as parchmentization, to cement, more or less perfectly, the separate fibres, and thus form a more or less perfect mechanical and chemical union between the interlaced fibres.

3) Carbons made from wood by simply splitting the wood in a direction as nearly coincident with the direction of the fibres as possible.

4) Carbons made from either of the above, but,..exposed to the flashing process of hydrocarbon treatment.

5) Carbons made from a perfectly homogeneous base...

"To those engaged in the manufacture of lamps, it is well known that the structure of the carbon resulting from the use of any one of these primary bases, or substances, out of which the loop is first cut, is precisely the same as the original base.

"Thus, if paper be used as the primary base, every fibre and every defect existing in the paper will exist as perfectly after carbonization as before. Examination under a high power lens will disclose the criss-cross, interlaced arrangement of the fibres, and the fibres will be as perfect in form and arrangement as they were in the paper. The same is true of carbon made from any other material. That is to say, the carbons preserve with absolute fidelity the arrangement, structure and shape of the substances of which they are made. It follows, therefore, that every defect existing in the material of which the carbon is made, no matter how great or small, exists also in the carbon.

"Now, an untreated carbon, made from paper...is very defective. Defective, first, because the fibres are not bound together; and secondly, because the paper not being of uniform thickness, nor of uniform density, the carbon is also not of uniform thickness nor uniform density. Put such a carbon into an electric lamp, put the lamp in a circuit where you can control the strength of the current, turn on a very feeble current at first; if no part of the carbon begins to glow, increase the strength of the current very gradually, and you will notice that some part or parts of the carbon begins to glow before the remainder shows any visible signs of heat. Increase the strength of the current again, and you will generally find that some part or parts of the carbon are vividly incandescent before the other parts are above a dull red.

"Now, what is true of the carbons made from paper, is more or less true of carbons made from cotton thread, linen thread and silk thread.

"...I met with these very difficulties in trying to secure uniform carbons very early in my work on the incandescent lamp. And these difficulties appeared in a very exaggerated form, in the kind of carbon I first attempted to use, and with the means then at my disposal to secure them thin and long enough to meet my needs. To overcome these..set me thinking. I was very familiar with the process of gas manufacture, and had known for years of that bugbear of the gas engineer, namely, the deposition of solid carbon on the inside of gas retorts, resulting from the decomposition of the vaporous and gaseous hydrocarbons evolved during the destructive distillation of the coal. I was also aware of the fact that the deposit of solid carbon formed very irregularly inside the retort, and seemed to be greatest where the temperature of the retort was highest. The thought occurred to me that if I placed my defective carbons in a carbon bearing liquid, vapor or gas, and then passed sufficient current through the carbon to at first raise the temperature of the defective spot to a red heat, the carbon would be deposited on the spots of highest electrical resistance and build them up, thus making the very agency potent in the destruction of the carbon serve the purpose of building up and obliterating the defective spots. It worked, and in a short time I brought the hydrocarbon treatment to a degree of perfection which left little more to be done...So with the electrician, if he wants uniform lights, he must have practically the same amount of matter in each lamp, and the resistance of the carbon must be substantially the same. With the hydrocarbon process this was easily accomplished by making the carbons all slightly smaller in cross section than was actually needed, and then depositing carbon on them in the manner described until the defective spots were all built up,and, continuing the process a little longer, it was quite easy to make the carbons all of substantially the same electrical resistance, so that, with a given electromotive force, substantially the same current would pass through all when placed in multiple arc; or, if placed in series, they would all have substantially the same electrical resistance, and, therefore, all require substantially the same expenditure of energy to bring them to the same degree of illumination."

[Note by the writer: Sawyer and Man were first to patent the hydrocarbon treatment process, but it appears Weston actually worked on the process before them. The Sawyer-Man application was submitted in Oct 1878 whereas Weston developed the idea in 1877.]

"So much for that class of carbons covered by the first division.

"I will now refer to the characteristics of the second class. In this class some of the defects, due to imperfect contact of the adjacent fibres, are overcome by reason of the fact that the parchmentizing process converts the outer layers of the fibres into a glutinous mass, which unites the fibres to form a more or less solid body of cellulose. But this parchmentization of the outer surface of the fibres does not wholly destroy the fibrous character of the mass, nor does it make the cross section uniform throughout the length of the loop. It follows, therefore, that carbons made from this class of bodies are still very imperfect, and exhibit in use some of the serious defects of the carbons of the first class. Hydrocarbon treatment is nearly as essential with these carbons, to secure satisfactory results, as with carbons of the first class, and that because of the existence of defective spots, as well as the necessity of equalizing the resistance of carbons. The parchmentization process appears to yield better results with thread loops than with loops made from paper.

"We have now come to that class of carbons made from substances I have placed under the third head...They ...are prepared from a substance found in nature, such as wood, bamboo, cane and other similar substances, the fibres of which are more or less parallel and continuous and bound together by the natural cementing substance between the fibres. The principal objection to these substances as the base for making carbons for incandescent lamps arises from the fact that it is almost impossible to get two samples of wood having the same density, and the great difficulty in splitting the wood into thin long lengths in planes parallel to the fibres.

"Mr. Edison showed excellent judgment in selecting bamboo as the base from which to make carbons for his lamps. For of all the woods available it is probably the best for the purpose. Its fibres are very straight, it is quite close grained and splits admirably. Not withstanding these advantages, it is not by any means perfect, and there has been much complaint regarding lamps made from it. Most of the lamps under proper test show defective spots in the loops, and few of them die as good lamps ought to. That is to say, they do not gradually die all over, so to speak, but fail at some particular spot which was initially weak.

"Moreover, the variation in electrical resistance of the carbons obtained from loops of the same cross section and length is very great even when great care is taken, and unless the carbons are submitted to some subsequent process after the first baking, it is practically impossible to obtain even moderately uniform results. It is not to be expected that it is possible to find a natural product like wood have a uniform density, or even approximate it. Consider for a moment the conditions of growth of plants. Are they not similar in many respects to human beings - products, to a large extent, of their surroundings? Is it reasonable to suppose that any two trees growing either close together or a long distance apart shall have the same properties exactly? I think not. In other words, there must of necessity be a considerable difference in the properties of the wood, due to climatic and other conditions, as well as those differences which are known to exist in the strength and character of the wood taken from the same tree.

"Carbons of the fourth class include carbons made from any of the other three classes already referred to, but subsequently subjected to the hydrocarbon process.

"There is no doubt about the general correctness...concerning the advantages of treated carbons over untreated carbons. I am quite sure of the absolute necessity of treating carbons of the first class and possibly of the second class, in order to get commercially useful results. I am quite sure that carbons of the third class, straight fibre, natural fibre, or bamboo type, or whatever you may choose to call them, are generally improved by the hydrocarbon process, properly applied.

"I believe enough has been already said on the effect of the physical characteristics of the base on the resulting carbon and its influence in determining the lifetime and efficiency of lamps to make it clear that what is really required to secure the perfect lamp is an absolutely homogeneous carbon of uniform cross section from end to end of the loop, the loop will not suddenly fail at a single point, long before the principal part has suffered sensible injury by use. Such a carbon will gradually die, so to speak, all over. It cannot fail at some sharply defined spots, because there is no defective spot to be raised to a higher temperature than the other parts of the carbon. Such a carbon, even if subjected to the most searching tests, will exhibit an absolutely uniform degree of incandescence from end to end of the loop, owing to the non-existence of defective spots. This kind of carbon may be raised to a much higher temperature than ordinary carbons, and so be much higher in efficiency, and still give a much greater average life than any other carbon I know of. To obtain such a carbon it is necessary, of course, to secure a base having also a perfectly homogeneous structure. I secured such a base many years ago in several ways, but after securing the base it took several years of work to overcome the difficulties encountered in making carbons from it. But the difficulties were all overcome, and the result amply repaid me for the labor and time expended. The best way of securing the base is fully described in United States patent No. 264,987, and dated September 26, 1882.

"The characteristics of the carbon and its advantages are set forth in another patent, issued to me on September 26, 1882, No. 264,986. Within the past year I have made many improvements in the method of making these carbons.

"Such a carbon requires no treatment to make it better adapted for use in an incandescent lamp. If properly made, it most perfectly fulfills all the requirements of the art as it exists to-day. It makes the most efficient and durable lamp known - the thing you want. It is cheap..."

As stated above, Weston received U.S. Patent 264,986 for a 'Carbon Conductor for Electric Lamps.' An excerpt is taken from that patent that describes the tamidine material:

"If cellulose - that is to say, cotton, cotton waste, linen, or paper - be subjected to the action of a mixture of nitric and sulphuric acid, the result is a substance which, though fibrous and possessing in some other respects nearly the same physical qualitites as the pure cellulose, differs radically from it, being explosive and burning without appreciable residue when out of contact with the air. This substance is commonly known as 'pyroxyline,' 'gun-cotton,' or 'nitro-cellulose.' By dissolving this with a mixture of ether and alcohol collodion is produced. By treating it with various other solvents - such as nitro-benzole, naptha, camphor, and other well-known solvents - the substance known as 'celluloid' is produced. Both collodion and celluloid may be formed in thin sheets and dried: but so long as the characteristics of the nitro-celluloe remain they are both unfit for the production of the carbons, for the reason that they burn without residue. In order, therefore, to render them suitable for my purpose, I deoxidize them so to speak, or, in other words, I treat them with such chemical agents as will deprive them of their nitrous qualitites and bring them back to the chemical condition of cellulose. Among such reducing agents may be mentioned ammonium sulphide, protochloride of iron, sulphate of iron, and others. The sheets of collodion or celluloid are immersed in a solution containing one or the other of the above-named agents and allowed to remain therein until they are entirely reconverted to their original chemical condition. In many respects they resemble closely the ordinary celluloid. They become transparent, very tenacious and flexible, and carbonize slightly less readily than ordinary cellulose. From these blanks or strips are cut or stamped having approximately the shape and size desired for the carbon conductors. They are then carbonized by being packed in a closed retort or muffle between plates of refractory material and exposed to a high temperature, the preparation of the carbons being in this respect substantially the same as that commonly followed in the production of carbon conductors from fibrous substances. After carbonization the strips may be mounted and inserted in the lamps in well-known ways, no further treatment being required."

Although Weston used the term "tamidine" in his U.S. patent 340,397, the origin of the word is not known by this writer. Some lamps with the tamidine filament can be readily recognized by the sinuous shape of the filament. The filament was made in this form to correct the poor light distribution given by a u-shaped filament with a rectangular cross-section. The sinuous shape also made the filament stronger. Some filaments were made with enlarged ends for connection to the lead wires with steel machine screws, nuts and washers. Later Weston developed a method of increasing the size of the filament ends by the deposition of carbon (U.S. Patent 340,397). Arthur Bright, in his book, claimed that the tamidine filament was used in the Westinghouse Stopper Lamp of 1893. A picture of some Weston lamps manufactured during the 1880s is presented below. It will be noticed that the filament attachment scheme in the two lamps to the left is different from the two on the right.