Resistance of Compound Conductors. 477 



In view of the statement * " that the coefficient of mutual 

 induction is less in iron than in copper wires," I may mention 

 an experiment made with the aid of one of the compensators, 

 in which the effect of the substitution of iron for copper is 

 directly examined. The mutual induction measured is that 

 between two circuits, one of which was composed of the two 

 copper coils of 21 and 22 convolutions spoken of above 

 connected in series; and the other of a single turn of wire 

 situated midway between, and lying in a shallow scratch or 

 groove on the wooden cylinder, by which its position was 

 accurately defined. The arrangements being the same as in 

 the determination of the constant of the compensator, the 

 value of the double induction (obtained by reversal) between 

 the circuit of a single turn of copper wire and the circuit of 

 43 turns was 40°* 7. The single turn of copper wire was now 

 replaced by a turn of iron wire of equal diameter and bedded 

 in the same scratch, with the result that the double induction 

 was 40 o, 6, the same value being obtained whether the iron 

 wire were included in the primary circuit with the battery and 

 interrupter, or in the secondary with the telephone. Care 

 had, of course, to be exercised in the disposition of the leads, 

 in consequence of the use of a single turn only for one of the 

 circuits. So far as the experiment could show, the induction 

 is absolutely the same, whether the single turn be of iron or 

 of copper. 



To return now to the bridge arrangement, the following 

 are a few examples of the use of the original form of appa- 

 ratus. The scale of the wire readings was in ^ inch, the 

 whole length (Q + R+S) being 1960. In ohms the resist- 

 ance of the whole wire is 4*00. The interrupter was the "reed," 

 making about 1050 (complete) vibrations per second. Thus 



^ = 2ttx1050. 



The first case is that of a helix of insulated copper wire, 

 without core of any kind. To get a balance the compensator 

 had to be placed at 54°, so that M = 36°, each degree re- 

 presenting 776 centim. The resistances also necessary were 



Q = 610, 11=190; therefore S = 1160. 



They are expressed in scale- divisions, the value of each 



of which is 



4*00 x 10 9 a _ . .^e centim. 

 = 2*04 x 10 



1960 sec. ' 



If, as we are almost entitled to do, we assume that the resist- 



* Proc. Roy. Soc. vol. xl. p. 468. 



