162 DOVE ON THE ELECTRICITY OF INDUCTION. 



all however during its continuance, may be expressed more 

 generally in the following terms : a primary current induces, 

 as long as its intensity is on the increase, a secondary current 

 in an opposite direction ; as long as it is decreasing, it induces 

 one in a like direction. If the term secondary current be ap- 

 pUed to the current induced by a primary current in a wire 

 parallel to, but not connected with it, and extra current to the 

 secondary current produced in a spiral connecting wire with 

 or without an iron core, by the action of each separate coil 

 upon that which lies next to it, if therefore this extra current 

 is considered as a particular kind of secondary current, in which 

 the same wire is the medium for the passage of the primary 

 and the induced current, then the phaenomena which have 

 been discovered in the secondary current may with great pro- 

 bability be supposed to exist in relation to the extra current. 

 But the spark produced on breaking the circuit of a galvanic 

 battery is more intense when the circuit has been closed by a 

 long spirally coiled wire, than when the same has been effected 

 by a short straight wire, and powerful physiological effects 

 appear, particularly when this spiral wire surrounds a piece 

 of iron, which are not perceptible with short, straight connect- 

 ing wires. Faraday, who bases upon these phsenomena the 

 existence of the extra current, conceives therefore (§ 1104) 

 that corresponding effects will always ensue by means of a 

 spiral and an electro-magnet when the electromotor is closed. 

 These effects must cause in the first moment a resistance, there- 

 fore something that is opposed to the shocks and sparks. It is 

 difficult to invent means for proving the existence of such nega- 

 tive effects. Faraday therefore endeavours to prove them by 

 positive effects which are simultaneously produced in a secon- 

 dary connexion. Now, as in more recent experiments in this 

 department the real experimental difficulty is not done away 

 with, namely, the prevention of the extra cun-ent being excited 

 on breaking connexion, and as moreover no diminution of power 

 at the end of the extra current corresponding to the increased 

 intensity of the sparks and of the physiological action has been 

 proved for the extra current supposed at the commencement, 

 the following reseai'ches may be viewed as a fiUing-up of this 

 gap, inasmuch as, by their aid, that which was required has been 

 effected in so perspicuous a manner, that these experiments may 



