US1115480A - Arc-light carbon. - Google Patents

Description

H. AYRTON.

ARG LIGHT CARBON. APPLICATION FILED MAR. 7, 1914.

1,1 1 5,480,` Patented Nov. 3, 1914.

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' f? e@ W/ //%/////////Z/// MJm/ l HERTHA AYRTON, OF HYDE PARK, LONDON, ENGLAND.

ARC-LIGHT CARBCN.

Specification of Letters Patent.

Patented Nov. 3, 1914.

Application led March 7, 1914. Serial No. 823,123.

To all whom, it may concern Be it known that I, HERTHA AYRTON, subject of the King of Great Britain, residing at 41 Norfolk Square, Hyde Park, in the county of London, England, have invented certain new and useful Improvements in Arc-Light Carbons, of which the following is a specification.

This invention relates to negative carbons for arc lights.

It has not yet been possible to produce a negative carbon which may be of any required thickness and which will maintain a perfectly steady arc with a suitable current and positive carbon under the varying conditions which exist in different lamp-houses or inclosures employed for arc lighting.

In the drawings accompanying this speciication, Figures l and 2 are longitudinal sections of electric light carbons which have been heretofore used, and Fig. 3 is a longitudinal section of an electric light carbon made in accordance with my present invention.

It is the object of this invention to provide a negative carbon which will fulfil these Conditions or which can be readily manufactured to suit any particular conditions.

It is well known that to secure a steady silent arc, giving constant illumination, it is necessary that the end surface of the burning tip of the negative carbon shall be so small that the arc cannot wander about on it. To overcome the various difliculties encountered with negative carbons it has hitherto been proposed to employ very slender carbons and to coat them with copper to increase the conductivity of the carbon and thereby prevent its becoming unduly heated by reason of its reduction in size. I have also previously proposed for the purpose of obtaining a slender carbon to employ a carbon consisting of a soft core surrounded by a shell of hard carbon covered with thin copper which by increasing the conductivity of the rod allowed of a much slenderer carbon with the same current than could otherwise be employed. Carbons so made give very good results for searchlights, where the inside of the lamp-house may be kept very hot, because the arc is always at a fair distance from the mirror of the lens. I found that under these conditions the carbons burnt away somewhat in the form indicated in Fig. 1 of the accompanying drawings, from which it will be seen that the soft core a burns away with a central depression in such a manner as to leave only a small ring A of hard carbon for the arc to rest upon. If, however, a carbon made as above described is used in a lamp-house which has to be kept comparatively cool, to avoid the cracking of the lens, I nd that the burning away of the soft corecannot always be depended upon, and at times it may only burn away at about the same rate as the outer shell of hard carbon b with the result that the tip of the negative carbon has no depression in it, and presents a blunt point entailing a large end surface as indicated in the dotted lines B Fig. l, which form is very undesirable and produces an unsteady and noisy arc. It will therefore be seen that in the case of ordinary carbons it has been proposed for the purpose of obtaining a slender carbon to employ in one case a substantial thickness of copper covering, and in another case to' employ a thin copper covering upon a hard shell inclosing a soft core. Further it has also been proposed to employ a thin copper covering to prevent the disintegration of carbons made of luminiferous material which burn in a different way from ordinary carbons, but none of these pro posals have been entirely successful in accomplish the object of this invention. It has also been proposed to make negative carbons wit-hout a copper covering and in which the core a, Fig. 2, was hard, or less easily combustible than the body of the carbon b so that a well tapered point may be secured, but such carbons have the disadvantage that the soft outer shell burns away too rapidly, and leaves a long narrow portion consisting mainly of a hard core (Fig. 2) which some- -times even breaks off at the holder c Fig. 2.

It has also been proposed to employ a carbon made of a hard core, a more easily combustible shell and an outer coating of copper, the latter being employed for the purpose of increasing the conductivity of the carbon and. therefore necessarily being of some substantial thickness. The copper covering employed by me is, however, a mere ilm serving only as a means for predetermining or adjusting the rate at which the outer shell will burn away.

I find that the desired results are obtained with a negative carbon lconsisting of the combination of the following: 1. A centi'al core of hard carbon. 2. A114 outer shell The thickness of the hard core and outer shell must of course depend upon the cur rentl that is required and I find that the thickness of the copper skin may also be varied with advantage Within certain limits. The lower of these limits is determined by the thickness required to withstand the necessary handling and packing; the upper limit depends upon the thickness which will result in the outer shell not burning away sufficiently far to allow 'the core to protrude well beyond it.l Within these limits quite a small variation in the thickness of the metallic skin makes a suiicient variation in the length of the carbon tip exposed to cause a considerable change in the rate of burning of the carbon. The thinner the copper all other things being equal the longer is the exposed tip and the faster therefore does the carbon burn. This leads to the following important result: I find that by choosing the right thickness of core, outer shell, and copper skin, it is always possible to obtain a negative carbon that will burn steadily at substantially the same rate as any positive carbon suitable for the current and pressure employed.

A negative carbon constructed according to this invention is shown in Fig. 3 of the accompanying drawings. There is a central core a of hard carbon, an outer shell b of carbon which is made to burn away more quickly than the core and a thin incasing lm c of copper, or other metal.

The copper coating here employed is not used, as such coatings have hitherto been, to reduce the size of the carbon by-increasing its conductivity.- So long as the carbon burns with such a tip that the core protrudes wellv beyond the softer outer shell, its size elsewhere is unimportant. The sole object of the copper film is to form a tightly adhering incombustible skin over the carbon,which, while protecting the major porouter shell vand a such as 1s describe tion of it fromburning, shall always melt off and volatilize at such a distance from the tip as `to allow the point to be well tapered, and to insure that none of the melted or volatilized metal shal'l enter the arc. A carbon with ahard core, a more'combustible protective metallic skin,

above, burns with perfect steadiness during the'whole of its life. Also there is so little copper thatmost of it volatilizes as it melts and does not drop oit' or splash the lens or mirror as is the case where lthick 'copper coverings are employed and the copper melts at such a distance along the carbon that'none vof, themelted or volatilized vmetalfenters the arc. Fig. 3 of the drawings shows ,'anexample'of the shape assumed during burning of a negative carbon made in accordance .with this invention.

Theouter shell and the central hard core need not necessarily -be'in engagement since a film or skin of copper may be'interposed between theml i I find that in a comparatively cool lamphouse with a current -fof 85 amperes, a 15 mm. negative carbon having a hard core 4 mm. in diameter, and a skin or coating of copper .02 mm. in thickness burns perfectly steadily with a. cored positive carbon 25 mm.` in diameter during the whole of both of their lives, and they both burn at substantially the same rate.

Other metals besides copper may be employed to form the incombustible coating, though copper has so far been found the most useful.

What I claim and desire to secure by Letters Patent is 1. A negative carbon for arc lights consisting of the combination of a hard core, a softer or more readily combustible shell and an outer film of copper or some other metal, the whole being such that the metal, while forming an adherent incombustible skin over the carbon, melts and volatilizes at such a distance from the tip that the melted and volatilized metal is too far from the arc to enter it, and that the b ared carbon burns so that the core always protrudes well beyond the outer shell and the arc remains steadily on the protruding end without moving substantially as set forth.

2. A negative carbon for arc lights, consisting of the combination of a hard core, a softer or more readily combustible shell and an outer film of copper or some other metal, the whole being such that the metal, while forming an adherent incombustible lskin over the carbon, melts and volatilizes at such a distance from the tip that the melted and volatilized metal is too far from the arc to enter it, and that the bared carbon burns so that the core always protrudes well beyond the outer shell and the arc remains 115 steadily lon the protruding end without moving, the parts of saidcarbon being so proportioned that it will burn away at the same rate as any positive carbon suitable for the current employed substantially as set 120 forth.

3. A negative carbon for 'electric lights comprising a hard core, a softer or more readily combustible shell and a film of incombustible f'volatilizable metal disposed 125 upon such slfell.

4. A neg tive carbon for electric lights comprising a hard core, a softer or more readily co bustible shell and a film of incombustibl volatilizable metal disposed 130 upon such shell, the Various parts of the my name in the presence of two subscribing structure belng of such relatlve proportlon Witnesses. that the same W111 burn away at the same HERTHA AYRTON. rate of speed as any posltlve carbon sultable 5 for the curlent employed, substantially as Witnesses:

set forth. G. V. SYMEs,

In Witness whereof I have hereunto signed 'i ALFRED B. CAMPBELL.

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