528 JAMES CLEKK MAXWELL. 



quantity itself electromagnetic momentum. Maxwell's con- 

 ception of the physical nature of this quantity will be 

 described presently. 



The determination of the laws of self-induction in electric 

 currents is another of Faraday's many contributions to 

 electrical science. After one of the Friday evening lectures 

 at the Eoyal Institution, a certain Mr. Jenkin informed 

 Faraday that when he broke the connection of the circuit in 

 his electromagnet by separating two pieces of wire which he 

 held in his hands, he felt a smart shock. Faraday said that 

 this was the only suggestion, out of a very great number, 

 made to him by ordinary members of a popular audience 

 which ever led to any result. On investigating the matter, 

 Faraday found that when a current is flowing in a coil of wire 

 if the battery be removed there is a tendency for the current 

 to continue after the removal of the battery, and that this 

 tendency is increased by increasing the number of turns of 

 wire in the coil, and still more so by inserting soft iron in 

 the centre of the coil. This tendency does not depend so 

 much on the length of the wire as upon the relative positions 

 of its parts, and if the wire be first doubled and then wound 

 into a coil the tendency disappears. If a few Grove's cells 

 send a current through a short straight piece of wire and 

 the circuit be broken a very feeble spark will be seen on 

 breaking, but if a large electromagnet be introduced into the 

 circuit a very much brighter spark will appear on breaking 

 contact, though the current sent by the battery is feebler. 

 Thus, when a current flows in such a coil its behaviour re- 

 minds us of that of water flowing in a pipe which, when an 

 obstruction is suddenly introduced so as to stop the flow, 

 exerts an enormous pressure for a short time upon the pipe 

 and obstruction, in virtue of the momentum which the water 

 has acquired ; but that the action is not due to any momen- 

 tum actually possessed by the moving electricity is shown 

 by the fact that it depends on the configuration of the wire. 

 This property of a coil is called self-induction. If the poles 

 of an electro-magnet be joined by a wire of great resistance 

 as well as by the battery, when the battery is removed a 



