62 



Mr. J. H. Poynting on the 



( Jenkin, p. 249). In the last column are the products of the 

 numbers in the two preceding columns. 



Metal. 



Distance 

 from point of 

 no induction. 



mM. 1 —n'M 2 , pro- 

 portional to 



E. 



(wM r wM 2 )E. 



Silyer 



Gold 



125 

 117 

 112 

 100 



80 

 74 

 45 

 38 

 35 

 10 



178 

 135 



116 

 84 

 50-1 

 446 

 22-46 

 18-87 

 17-35 

 5-75 



•21 



•27 



•375 



•21 



•72 



1-70 



1-25 



25 



4-5 

 16-8 



37-4 

 36-5 

 435 

 176 



361 



75-8 

 28-1 

 47-2 

 78-1 

 96-6 



Aluminium... 

 Copper 



Zinc 



Tin 



Iron 



Lead 



Antimony 



Bismuth 



Mercury has been omitted, as it gives a very much higher 

 value than any of the others. Were the induced currents in 

 the induction-balance proportional to the resistances given in 

 the table, the numbers in the last column would of course be 

 all the same. The deviations from equality are far greater 

 than could be accounted for by errors in the approximations I 

 have adopted, especially for the metals not at the beginning 

 or end of the list. Hence we are driven to conclude, either 

 that the resistances of the metals given in the tables are not 

 the same as the resistances of the metals used by Prof. Hughes, 

 or that the induced current is not proportional to the conduc- 

 tivity of the metal. 



It should be noticed that the method of measuring currents 

 by the sonometer assumes that the telephone integrates, as it 

 were, the current ; i. e. the loudness of the sound depends only 

 on the total current, not on the time during which the current 

 is passing, provided that the time be very short. I do not 

 know whether this point has been investigated ; but if not, it 

 would probably be easy to examine it by means of the sono- 

 meter. It would be advisable to modify the instrument in 

 such a way that the formulae might be more easily employed, 

 and that the approximations might be nearer to the truth. 



The formulae used in this paper may be obtained as follows, 

 the method being adapted from that given in Maxwell. 



The potential of a circular unit current at any point is the 

 same as that of a magnetic shell of unit strength bounded by 

 the circuit. This, again, is the same as the attraction of a thin 

 plate of matter of unit surface-density in a direction perpen- 

 dicular to the plane of the plate. If co be the attraction of a 



