Nxro. 28. 1878] 



NATURE 



79 



arre separated by a partition of kaolin or other similar 

 insulating materiaL 



I have thought it well to describe, as nearly as possible 

 in the words of the inventor, the electric candle, which is 

 Eow the subject of so much attention in its application to 

 electric lighting ; so that its relation to what follows may 

 be more clearly perceived. A remarkable peculiarity of 

 the direct current in electric lighting is that of its con- 

 suming the positive carbon at twice the rate of the 

 negative one, and while the negative carbon is a pointed 

 cone, like that of a pencil, the positive pole takes the 

 form of a hollow cavity or crater. 



M. JablochkofTs early experiments seem to have been 

 made with the direct current, and hence his carbons are 

 described as being of unequal thickness, in order that 

 the positive and negative carbons of the candle might be 

 evenly consumed. When the alternating current is used 

 for producing electric light both carbons are of the same 

 thickness, and are consumed at an equal rate, and both 

 points terminate in regular cones. This property of the 

 alternating current, besides other advantages, always 

 maintains the luminous point in the focus of any optical 

 apparatus used in connection with it, that is, when the 

 carbons are placed end to end, as I had occasion to point 

 out in a former paper read before the Society in 1873, on 

 an electro-magnetic induction machine for producing 

 alternating currents. 



M. Jablochkoff, in the course of his experiments, would 

 appear to have met with some difficulties in adapting the 

 direct or continuous current to a system of lighting with 

 his electric candles, and now uses the alternating current 

 for this purpose. The candle has also been simplified by 

 substituting a slip of plaster of Paris for the cartridge 

 and partition of kaolin formerly employed. 



To produce the alternating currents, however, to supply 

 a number of lights, it was found necessary to employ 

 powerful electro-magnetic induction machines, excited by 

 the currents from other smaller machines, according to 

 fhe principles laid down in my paper read before the 

 Royal Society, and published in the Philosophical Trans- 

 actions of 1867. From sixteen to twenty lights are pro- 

 duced from one of these electro-magnetic machines, each 

 light absorbing about one-horse power. 



The system of electric lighting above described would 

 now seem to be definitely established in some places as a 

 substitute for gas, and visitors to the French capital 

 -during the present summer will have been struck with 

 the fine effects produced in the avenues and squares 

 where the light is displayed. 



My connection with the history of this system of 

 lighting placed me in a position to make some experi- 

 ments with the Jablochkoff candle, and led to the dis- 

 covery of the following facts. One of the conditions 

 necessary for producing a constant light from the candle, 

 in its most recent form, was that the quantity and inten- 

 sity of the alternating current should be such that the 

 carbons consume at a rate of from four to five inches per 

 hour. If the electric current were too powerful, the 

 carbons became unduly heated, and presented additional 

 resistance to the passage of the current; the points at 

 the same time lost their regular conical form. If, on the 

 •other hand, the current were too weak, the electric arc 

 played about the points of the carbons in an irregular 

 manner, and the light was easily extinguished by currents 

 of air. 



In the course of these experiments I was struck with 

 the apparently insignificant part which the insulating 

 material played in the maintenance of the light between 

 the carbon points ; and it occurred to me to try the effect 

 of covenng each of the carbons with a thin coating of 

 hydrate of lime, and mounting them paraUel to each 

 other m separate holders, and without any insulating 

 material between them. The use of the lime covering 

 w-as mtended to prevent the light from travelling down 



the contiguous sides of the carbons. On completing the 

 electric circuit the light was maintained between the two 

 points, and the carbons were consumed in the same 

 regular manner as when the insulating material had been 

 placed between them. 



Two plain cylindrical rods of carbon three-sixteenths 

 of an inch in diameter and eight inches long, were now 

 fixed in the holders parallel to each other, and one-eighth 

 of an inch apart. The strength of the alternating cur- 

 rent was such that it would fuse an iron wire 0*025 of a" 

 inch in diameter and eight feet in length. On establish- 

 ing the electric current through the points of the carbons 

 by means of a conducting paste composed of carbon and 

 gum, the light was produced, and the carbons burnt 

 steadily downwards as before. 



Four pairs of naked carbons mounted in this manner 

 were next placed in series in the circuit of a four-light 

 machine, and the light was produced from these carbons 

 simultaneously, as when the insulating material was used 

 between them. The light from the naked carbons was 

 also more regular than that from the insulated ones, as 

 the plaster of Paris insulation did not always consume at 

 the same rate as the carbons, and thereby obstructed the 

 passage of the current. This was evident from the rosy 

 tinge of the light produced by the volatilisation of the 

 calcium simultaneously with the diminution of the bril- 

 liancy of the light from the carbons. 



The only function, therefore, which the insulating mate- 

 rial performs in the electric candle, as shown by these 

 experiments, is that it conceals the singular and beautiful 

 property of the alternating current to which I have directed 

 attention. 



As I have already said, the strength of the alternating 

 current must bear a proper proportion to the diameter of 

 the carbons used, and when a number of such lights are 

 required to be produced in the same circuit, the quantity 

 and property of the current will remain constant, while 

 the tension will require to be increased with the number 

 of lights. 



This simple method of burning the carbons wiU, I 

 believe, greatly further the development of the electric 

 light, as the carbons can be used of much smaller dia- 

 meter than has hitherto been possible. They may also 

 be of any desired length, for as they are consumed they 

 may be pushed up through the holders without interrupt- 

 ing the light. One of these developments will be a better 

 method of lighting coal and other mines. In this appli- 

 cation the alternating currents or waves from a powerful 

 electro-magnetic induction machine may be used for 

 generating, simultaneously, alternating secondary cur- 

 rents or waves in a number of small induction coils, 

 placed in various parts of the mine. The light may be 

 produced in the secondary circuits from pairs of small 

 carbons inclosed in a glass vessel having a small aperture 

 to permit the expansion of the heated air within. Dia- 

 phragms of wire gauze may be placed over the aperture 

 to prevent the access of explosive gas. By generating 

 secondary currents or waves without interrupting the 

 continuity of the primary circuit, the contact-breaker is 

 dispensed with, and the subdivision of the light may be 

 carried to a very great extent. 



A STUDY IN MAGNETISM 



THE name of Faraday will go down to posterity fore- 

 most amongst the names of the scientific men of 

 this century, for the simple comprehensiveness and 

 original beauty of his researches in electricity and mag- 

 netism ; chiefly, perhaps, for his discovery of magneto- 

 electricity — the kind of electricity that can be induced in 

 conductors which are caused to pass near magnets. 

 Those who have carefully read Faraday's works know 

 how he was led to this discovery by the conception he 

 had formed of magnetic force. LFntil his time magnetic 



