114 DOVE ON THE ELECTRICITV OF INDUCTION. 



I. Currents induced by the evanescent magnetism of electro-mag- 

 netized rods of iron and bundles of wires, when the magtie- 

 tizing current ivas that of a galva?iic battery. 

 1. Cornparison of the galvanometric and physiological action. 

 9. If a solid rod of iron is placed in one of the spirals of 

 the differential inductor, and a bundle of wires in the other, so 

 that equilibrium is established as regards the galvanometer, 

 these cuiTents, which galvanometrically compensate each other, 

 produce powerful shocks upon the human body when it is made 

 to form part of the circuit. By diminishing the number of wires, 

 these shocks may be reduced to nothing, but then the currents, 

 which physiologically compensate each other, cause a powerful 

 deflection of the galvanometer needle in favour of the solid cy- 

 linder. How great this difference is, may be seen by one of the 

 series of experiments with wire of a line in thickness. The num- 

 ber of wires requisite for compensation was as follows : — 



For the For 



galvanometer, sensation. 



Forged ii'on 110 -|- «■ 15 



Gray iron from the crucible furnace . . 92 24 



Soft steel 91 9 



Gray iron from the cupola furnace, with 



hot blast 45 18 



White iron from the cupola furnace, with 



cold blast 43 8 



White iron, crucible cast 41 10 



Hard steel 28 7 



Gray iron from the cupola furnace, with 



cold blast 27 11 



With forged iron, the number of wires that could be placed 

 in the wooden screw was insufficient for compensation in the 

 galvanometer. Without exception, therefore, the number of 

 wires required to compensate a solid rod of iron is greater 

 when the currents act upon the galvanometer than when they 

 act upon the body ; or in other words, when the currents are of 

 equal intensity as regards the galvanometer, the shock produced 

 by the bundle of wires is greater than that produced by the solid 

 bar of iron. To test this result in another manner, the follow- 

 ing experiment was made : — A differential galvanometer with 

 two equal wires, each of which made 100 revolutions round its 

 frame, was so connected with the induction-spirals, wdiich had 



