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LXVIII. Intelligence and Miscellaneous Articles. 



ON THE EFFECTS PRODUCED IN A VACUUM BY THE CURRENT OF 

 THE GRAMME MACHINES. BY M. JAMIN. 



AS soon as Davy had discovered the electric arc he hastened to 

 reproduce it in an enclosure void of air, in order to avoid the 

 combustion of the carbons. He saw that it could be enlarged, that 

 the carbonaceous material was carried from the positive to the nega- 

 tive pole, that the former was more luminous than the latter, but 

 that the general aspect of the phenomenon did not change. Des- 

 pretz repeated the experiment with a more energetic battery, and 

 ascertained that the carbon volatilized under the electric action was 

 deposited on the sides of the glass balloon as soon as the intensity 

 of the current exceeded a certain limit. It is probable that this 

 deposition of carbon takes place in all cases, but does not begin to 

 be ascertainable until the intensity is very great. 



Things happen quite otherwise when instead of a battery a 

 Ruhmkorff induction-coil is employed. It is known that it gives 

 rise to successive induction-currents in opposite directions : the 

 first currents, inverse, have too little tension to clear the distance 

 separating the points, and produce nothing; the second, direct, 

 acquire at last an enormous tension, but have little duration and 

 transport but a slight quantity of electricity. The whole therefore 

 reduces to successive almost instantaneous discharges separated by 

 comparatively prolonged intervals of rest, to sparks, and in vacuo to 

 emanations of light ; but there is never an electric arc, because the 

 quantity of electricity is insufficient. 



G-ramme machines with alternating currents participate at the 

 same time in the properties of batteries and the induction-coil, 

 while presenting some special characters : they give two alternately 

 contrary currents ; but these are equal, last the same time, are 

 separated by only a brief interval, and transport, on account of 

 their duration, a large sum of electricity. They could be replaced 

 by a battery the direction of which changed periodically and with- 

 out interruption ; but they differ from a battery by the great inten- 

 sity of their currents : when one of these is ending it is augmented 

 by the extra current ; when commencing, it profits by the inverse 

 electromotive force of the preceding current, as I have demon- 

 strated*. In short, the effects will be those of batteries with an 

 aggravation due to the enormous tension, and those of the induc- 

 tion-coil with the aggravation resulting from a larger quantity of 

 electricity. This is verified by experiment. In air, instead of a 

 single are, several can be lighted; and the number of them is mul- 

 tiplied by increasing the tension — that is, by diminishing the sec- 

 tion and increasing the length of the induced wire as well as the 

 velocity of the rotation. This has permitted me to maintain sixty 



* Comptcs Itendus, t. xcii. p. 1201. 



