Resistance of Compound Conductors. 479 



ferential secondary currents induced in it altered the readings 

 with variable currents to 



Q=660, 11=190; M = 29±°. 



The effective self-induction is evidently diminished, and the 

 effective resistance increased in accordance with the universal 

 rule *. The precise values may be obtained from our two 

 fundamental equations, in the manner exemplified above. 



If the foregoing experiment (with the copper core) be 

 attempted with the scraping-contact interrupter, giving a 

 mixed sound, no definite balance is obtainable. 



The second example that I shall give is of a wire of soft 

 iron about 1| metre long and 3*3 millim. diameter. Here 

 with variable currents from the reed-interrupter, of the same 

 period as before, 



Q=178, R=190, S = 1592; M = 8x 776 centim. ; 

 from which we find 



P= -985 5^? =20-93 scale-divisions. 



In the present case the ordinary simple rule (QR/S) would 

 lead to an error of 1^ per cent. only. 



The resistance to steady currents is given by 



_, l oo x 190 ; fl 



Po= 1670 =1138 ' 

 We may conclude that the effective resistance to variable 

 currents of this frequency (1050) is 1*84 times the resistance 

 to steady currents. 



A long length of wire from the same hank was examined 

 later by another method (p. 488), and gave for the ratio in 

 question 1*89. 



In some of his experiments f Prof. Hughes found that it 

 made but little difference to the self-induction of an iron wire, 

 whether it was arranged as a compact coil of several turns, 

 or as a single wide loop. The question is readily examined 

 with the present form of apparatus ; for, since the resistance 

 is not altered, the compensator readings give an accurate 

 relative measure of the self-induction. A. hank of nineteen 

 convolutions of insulated soft iron wire required for balance 

 25 0, 8; but when opened out into a single (approximately cir- 

 cular) loop, the reading was only 11°'2, a much greater differ- 

 ence than that mentioned by Hughes. 



* See equations (8), (10), (11), (12), Phil. Map:. May 1886, pp. 373- 

 375. 



t Proo. Roy. Soc. vol. xl. p. 457. 



