of the Induction Current. 7 



ance and colour of the light when the receiver was alternately- 

 touched on the positive and on the negative side of the apparatus. 

 Its colour is whitish throughout. 



This side light, as I will call it, as it is obviously analogous to 

 the lateral emissions which have long been known in powerful 

 electric discharges in free air, was most intense when the induc- 

 torium was made to give sparks in the air, and at the same time 

 the air surrounding part of the joining partially exhausted. The 

 further apart the poles are moved, the more intense is the light, 

 and of course the tension upon the wires. It is, on the other 

 hand, relatively feeble if the poles of the apparatus are con- 

 nected with the armatures of a Ley den jar, in which, as is known, 

 recurrent currents are formed, and the tension upon the wires is 

 feebler. 



The side light is a useful indicator of the degree of free elec- 

 tricity on the wires. 



If, for example, an induction coil is placed in metallic connexion 

 with a larger one, and an induction current is alternately pro- 

 duced in the first and in the second, while the other is used 

 empty as an accessory coil, even the sensation of feeling shows 

 that the tension in the wire is stronger when the current is in- 

 duced in the smaller coil, although the current is feebler in that 

 case than in the other; but this is shown much more convin- 

 cingly by the side light. The strengthening action produced by 

 introducing a bundle of iron wire into the accessory coil cannot 

 be more surprisingly shown than by the side light. 



I must in conclusion mention that in 1859, Koosen*, on the 

 occasion of another investigation, made observations which are 

 closely allied to mine, but do not quite coincide with them. He 

 observes the phenomenon in a form in which it is essentially a 

 phenomenon of partial currents. He offers, that is, two paths for 

 the induction current, one through air and one through metal, 

 by letting the poles of the instrument give sparks, and joining 

 them at the same time by a very long wire, in which he finds 

 that the striking-distance of the sparks, in spite of this metallic 

 lateral circuit, is either not at all or not perceptibly diminished. 

 The free electricity in the wire has not indeed escaped him ; but 

 since he only states that the wire has a certain tension which can 

 be shown by the gold-leaf electrometer, while he does not mention 

 the sparks and their piercing action, he has probably not seen 

 the phenomenon in its full development, perhaps because he 

 used covered and varnished wires, perhaps because he only studied 

 it at one branch of the current. Finally, he does not dwell 

 upon the cause of the phenomenon. Although he has in all pro- 

 bability used the wire in the form of coil, yet he does not say 

 * Pogg. Ann. vol. cvii. p. 211. 



