324 M. P. Riess on the Passage of Electric Currents 



was only partially siuTOunded by a blue light, that is to say, it 

 extended only over the anterior hemisphere of the knob or the 

 anterior surface of the disc. ^Vhen the current was increased, the 

 blue light was confined to a still smaller space, and was momen- 

 tarily hidden by the broad reddish sheaf of light which extended 

 between the electrodes. When these luminous phjenomena have 

 been once observed in the dark, it is easy to recognize them 

 in daylight by merely shading the cylinder, whatever connexion 

 the electrodes may have with the induction coil. 



The experiments with the Leyden battery serve to explain 

 the phfenomena of the induction apparatus. It has been shown 

 that the electricity in the battery generates a less amount of heat 

 in the discharge circuit when it is transmitted through rarefied 

 air from the small to the large surface. The same takes place 

 Avith the inteiTuption current of the induction apparatus. A 

 brass disc was placed in a glass cylinder at a distance of 1 line 

 from the small surface in the cover, and the cylinder, together 

 with the delicate electric thermometer, introduced into the cir- 

 cuit of the induction coil. The heat produced was in general 

 small, but considerably smaller when the direction of the inter- 

 ruption current was from the small to the large surface. When 

 the large disc was negative, the liquid in the thermometer sunk 

 from 1 to 3 lines ; w'hen that disc was positive, from 5 to G 

 lines. I obtained a constant result by allowing the current 

 passing from the great to the small surface to continue for some 

 time, until the liquid in the thermometer attained its lowest 

 position ; the sudden reversion of the current immediately caused 

 the liquid to rise. By simultaneously introducing a galvano- 

 meter into the circuit, it w^as found that the greatest heat was 

 produced when the deflection of the needle was smallest, and 

 the smallest heat when the deflection was greatest. With the 

 Leyden battery no doubt can be entertained that the change in 

 the heating efiect is occasioned by a change in the method of 

 discharge, that an increased heating effect is produced by con- 

 verting the slow glimmering discharge into the much more 

 quickly completed discontinuous discharge. If a similar change 

 in the discharge of the interruption ciirrcnt of tlie induction 

 apparatus be assumed, the cause of the changed action will be 

 the same, and that of the passage of the contact cm-rent will be 

 found. From experiments with the Leyden battery, it is known 

 that the conversion of the glimmering into the discontinuous 

 discharge occasions a less deflection in a galvanometer introduced 

 into the circuit, as well as a diminution in the quantity of any 

 substance decomposed in the circuit. Whilst the glimmering 

 discharge does not injure the electrodes, the discontinuous dis- 

 charge tears parts of them away, and scatters the incandescent 



