Soft Iron, especially with weaker decomposing-powers. 4>7 



outer side, were cemented; upon these a covered copper wire 

 (without its covering, 0*45 millim. thick ; with it, 0*67) was 

 wound as close and uniformly as possible. This wire, mostly 

 used as primary closing, had 800 turns ; it was connected with 

 a galvanic series. The mean contour of a winding is pretty 

 accurately represented by the combination of a rectangle 11*1 

 millims. wide, and 24*5 high, with two semicircles of 11*1 dia- 

 meter (PL III. fig. 1). Accordingly the quantity denoted by P 

 (upon the exact knowledge of which much less depends than 

 upon the determination of M) is found by calculation to be 3*87 

 millims. 



Equations (1) and (2), applied to the ring described, give us 

 the following formulae for the calculation of R and k : — 



1 T^ 



^-3-87 



320 n' i -p „ 



k = WT72 ; R = 16-84*. 



Upon the first layer of wire-turns, which mostly served alone 

 for the primary wire, 750 rounds of the same piece of wire were 

 wound, and likewise filled up the entire periphery of the ring ; 

 in this second layer, however, the wire was divided into five 

 separate portions, of which the number of circumvolutions were 

 respectively 50, 100, 150, 200, and 250. As required, one or 

 another of these divisions, or different combinations of them 

 were connected with a multiplier and used as a secondary wire. 

 The number of the secondary circumvolutions might thus be in- 

 creased, step by step, to 100, 150, . . . 700, 750; and, with the 

 same effective number of turns, I could vary the resistance of the 

 secondary wire, by using, for example, at one time the 50 divi- 

 sion alone, at another combining the 250 and 200 divisions in 

 opposite directions. All these wires were wound in such wise 

 that with each the longitudinal current, according to Ampere, 

 was compensated by a returning round of wire. (Fig. 2 is a 

 sketch of the inner wire.) 



With greater decomposing forces I could make use of a smaller 

 number of secondary turns. Then I had no need of the divi- 

 sions above mentioned for the secondary closing, and merely 

 caused the conducting wire of the multiplier to run round the 

 ring in ten turns. Those divisions, however, I could now coti- 

 nect with each other, all in one direction, and with the first 

 layer of wire; whereby the number of the primary circumvolu- 

 tions became 1550, and with the same galvanic series the de- 

 composing force was considerably greater. With this arrange- 

 ment, taking into account the alteration of n and P (with the 

 outer turns the length of the oval was 37*75 millims., the breadth 



