of Coefficients of Mutual Induction. 417 



induction. Whence the common magnetic induction is 

 10 7 x4*95 

 468x850" 

 This seems very small. But the dimensions of the armature 

 are such that it can have hardly any core. 



But I must not now pursue the subject of the dynamo. 



The method of the earth induction-coil and ballistic galva- 

 nometer is susceptible of numerous applications. I have in 

 my mind the direct determination of the capacity of a con- 

 denser, and a method following the lines of that used by 

 Weber, for the determination of resistance. He employed 

 earth induction-coils on this principle, though I do not know 

 how they were arranged in detail. I cannot conceive how a 

 tangent galvanometer could be employed for the ballistic 

 work, as seems to be contemplated in Maxwell's account. I 

 should determine the constant of the ballistic galvanometer 

 for the purpose by a shunt comparison with a tangent galva- 

 nometer. I believe Mr. Glazebrook has done something of 

 this kind in another case. In fact I have been through the 

 work of such a method ; but where there is a shunt to be 

 verified, in this case 1 : 10,000, as well as other determina- 

 tions of resistances, the errors of temperature are so trouble- 

 some to deal with, that in the present state of the subject I 

 doubt whether it is worth while to spend much labour on it. 



The numerical results of the experiments referred to are 

 appended. 



Coefficients of Induction of 250 and 100 coils. 



Feb. 21. Distance between central planes, 6 = 13*97 centims. 



by half turn of 1000 coil. 



Arrange- Galvano- 



ment. a. /3. 9. meter. M. 



1000 S 152 '* 5 164 '' 5 15 ° 45 ' 18 ""break 101 107x7 ' 6903 



Calculation by Maxwell's original formula . . . 10 7 x 6*164 



Feb. 22. Distance between central planes, 6=7*925 centim. 

 j3 by half turn of 1000 coil. 



Arrange- Galvano- 



ment. a. j3. 9. meter. M. 



1000 1 2 ° 3 '" 7 170 '' 12 ° 38 ' 18 m b k rea a k d 108x1 ' 2593 



•Calculation by Maxwell's original formula . . . 10 8 x 1*070 

 •Calculation by formula for dimensions of section of coils 10 8 X 1*104. 



* New edition of Maxwell's ' Electricity and Magnetism/ vol. ii. 

 pp. 314, 320. 



Phil. Mag. S. 5. Vol. 23. No. 144. May 1887. 2 F 



