MAGNETO-INDUCTION APPARATUS. 



647 



A coil (K K), designated the secondary spiral, hollow within, and consisting 

 of numerous turns of thin insulated wire, passes in the same direction as the spiral 



FIG. 226. I, Diagrammatic representation of the sliding electromotor of du Bois-Reymond. II, Key for tetani- 

 zation. Ill, Electrodes with mechanism for interruption. 



(x x) of the primary current. This is mounted upon a long board or slide^(p p), 

 provided with a scale upon which it may be moved over the primary spiral, 

 which it then receives into its concavity 

 (the induced current being then strongest) , 

 or it may be removed any desired distance 

 from the primary spiral (the current then 

 being feeblest) . The degree of separation 

 of the coils is thus an index of the strength 

 of the stimulus. The measurement of the 

 current-strength may naturally be made 

 more accurately by means of graduated in- 

 struments. According to the laws of voltaic 

 induction there develops in the secondary 

 spiral (K K) on closing the primary current 

 an induced current opposite in direction to 

 that of the primary current, and on closing 

 the primary current an induced current in 

 the same direction. Moreover, according to 

 the laws of magneto-induction the magneti- 

 zation of the iron bar (i i) within the pri- 

 mary spiral (x x) through closure of the pri- 

 mary current causes the development of a 

 current in the secondary coil (K K) in the 

 opposite direction, and the demagnetization 

 of the bar, by opening the primary circuit, 

 an induced current in the same direction. 

 These facts, explain the more powerful 

 effect of induced opening currents, as com- 

 pared to closing currents. The removal of 

 the inequality in the two currents has been 

 discussed on 'p. 646. 



The magneto-induction (or rotation) 

 apparatus (Fig. 227), devised by Pixii, and 

 improved by Saxton, and provided by 

 Stohrer with a commutator, consists of a 



powerful horseshoe steel magnet, opposite to whose two poles (N andS) is placed 

 a horseshoe of soft iron (H) , which can be rotated about a horizontal axis (a b) . 



FIG. 



227. Magneto-induction Apparatus 

 Stohrer's Commutator. 



with 



