225 



To eliininate the effect of (lamping due to eddy cau'rents two sugges- 

 tions have been made. (1) to insert in the galvanometer circuit a special 

 key so arranged as to break the galvanometer circuit a brief instant after 

 the charge has passed, thus securing always the open circuit conditions of 

 curve D ; (2) to insert a special key in the galvanometer circuit so ar- 

 ranged that the galvanometer will always be closed through a circuit of 

 constant resistance. Both of these methods are satisfactory, but only with 

 perfectly operating keys, which condition is not easy to secure. 



By far the safest and most convenient procedure is then to calihratv 

 the r/nlvanometer for the in-ecisc conditions under irhicJi it is to he used. 

 This may readily be carried out by permanently including in the galvano- 

 meter circuit the secoiidai-y coil of a standard mutual inductance, and by 

 simply reversing a known current in the primary circuit the constant can 

 bo accurately determined from the resulting throw. 



For a standard of mutual inductance it has long been customary to 

 rely on the long solenoid with a short coaxial solenoid for a secondary 

 coil. Unless these are well made, with exceptional care and by experienced 

 hands, they are by no means standard. Tlie writer lias measured the mu- 

 tual inductance of a large number of such coils from diffei'ent makers, and 

 the subjoined table will sJiow the discordances between measured and cal- 

 culated values for a few of them. 



The calculated values were all secured from the approximate formula 

 based on 4 - ni as the value of the field at the center of a solenoid, while 

 the measured values were obtained by Maxwell's method, by comparison 



[15—29034] 



