DOVE ON THE ELECTRICITY OP INDUCTION. 107 



the galvanometer least must possess an increased velocity in pro- 

 portion to its decrease of power. 



I shall now first illustrate the apparatus which w^as made use 

 of in the experiments by a simple figure, and then proceed to 

 the more accurate description of it. 



1. Principle of the Differential Inductor. 



1. When an electric current is excited in two similar wires, a b 

 and c d (Plate I. fig. 1), which are connected with each other by a 

 wire b c, this cui'rent will produce on being discontinued a second- 

 ary current in the same direction as the first, in two wires «/3 

 and y 8 placed parallel to the fii'st two. If these wares however 

 are connected crossways, i. e. (fig. 2) u with y and j3 with 8, then 

 the currents induced in a /3 and y 8 by the primary current a b will 

 flow in contrary directions, and if they are equal will completely 

 neutralize each other. But if at the side of a b a second closed 

 wire e/ff h (fig. 2) is placed, the current induced in it reacts 

 upon a b and « /3, and acts in a retarding manner as it is passing 

 in the same direction as the currents in ab and a. /3, i. e. in a 

 manner to weaken them for all the tests, w^hich, the quantity of 

 electricity being the same, are less affected when it traverses the 

 wire in a longer than when the same occurs in a shorter time, 

 therefore weakening with reference to the physiological action 

 and to the magnetizing of steel, whilst the effect upon the galva- 

 nometer and the property oi magnetizing soft iron are not changed 

 by it. The phaenomena of induction with relation to these tests 

 which arise from the presence of efg h in the wire « /3 y 8, after the 

 equilibrium of the current is destroyed, will appear from this to 

 be caused by a current passing in the direction from /S to «, as 

 the unretarded current induced m yl hy cd preponderates over 

 the current induced in a /3 by a 6, but which is retarded by efg h. 

 These phaenomena of induction must also be solely ascribed to 

 the action of efg h upon « /3, as the direct action of a b upon 

 a /3 is not lessened by the presence of efg h, as is obvious from 

 the principle of multiplication applicable in the case of induc- 

 tions with superimposed coils of wire. 



2. If instead of the endless wire efg h a rod of iron s n (fig. 2) 

 is substituted at right angles to the plane of the wire, it will be 

 magnetized by the primary current. The evanescent magnetism 

 of this rod of iron when the primary current a h is discontinued, 

 induces in like manner a current in «/3, which is moreover in a 



