Induction- Coil Potential* 

 Table VII. 



579 





A. 



v c . 



VVa. 



Case I 



Case II 



4-4 



6-0 



14-0 



71610 



87420 

 125700 



34130 

 33310 

 33590 



Case III 



To within about 2'5 per cent., therefore, the calculated 

 values of the maximum secondary potential are proportional 

 to tue values indicated by the electrometer. 



TVith regard to the estimation of the absolute values of 

 the potentials, no source of high potential suitable for 

 standardizing the electrometer was available, and recourse 

 was had to the method of spark potentials. It appears to 

 be agreed that the potential difference required to produce a 

 1-ceitimetre spark, in air at atmospheric pressure, between 

 ball electrodes 2 cm. in diameter, is not far from 31000 

 volts*. 



Tb above theoretical expression for V 2 gives in Case I. a 

 maxinum value of 31000 volts at i = 1*298 amperes, but 

 this i> calculated on the assumption that the inductances are 

 constmt and equal to their maximum values. At this 

 curreit, however, L 21 is (from Table I.) about '916 of its 

 maxinum value, and we may assume that L x and L 2 are 

 alterel approximately in the same proportion. Hence from 

 the expressions for n l} n 2 , A 1? and A 2 we find that V 2 is 

 approximately proportional, as far as small variations in the 

 permeability of the core are concerned, to the square root 

 of L 2 . Assuming this result, the calculated value of the 

 curreit required to give a maximum voltage of 31000 is 

 1-298 V'916, i. e. 1*356 amperes. 



Th< least current observed to give a 1-cm. spark in Case I. 

 was ]'36 amperes. This was repeated on many occasions 

 and vith both forms of interruptor, and found to be fairly 

 definre. Sometimes the spark would not pass at less than 

 1*38 nnperes, but on most occasions the result was as given 

 above 



In Case II. the current giving 31000 volts, calculated 



* Peydweiller (Wied. Ann. xlvm. 1893) gives 31290 volts, while ac- 

 cordiig'to C. Miiller {Ann. d. Physik, xxviii. 1909) the voltage is 30240. 

 An aicount of the results of different observers is given in Fleming's 

 'Priwiples of Electric Wave Telegraphy and Telephonv,' 2nd ed., 

 pp. 2(3-8. 



