Potential of an Induction- Coil at u make" 331 



the maximum secondary potential at make, is not great. 

 Measurements of the spark-length at make show that if R r 

 is reduced from a large value the potential at first increases, 

 but it reaches a limiting value after which further reduction 

 of Ri causes no change. 



(III.) Hi = 13 ohms, L 1 = 0-03625 henry, ^/L^ 358*4, 



2ttw=7174, &!= 591-1, 8 = 363, 



4> m = 1*629, Y 2m = 32580 volts. 



This case and the first illustrate the effect of reducing Lj to 

 one quarter, as by connecting the primary layers in series- 

 parallel -~C[]Cul}~CjuuD~ • -^ ne e ff ect i s to increase greatly 

 k x and 8, and n is appreciably reduced. <f) m is reduced, but 

 L 2 ]/L! is doubled, so that there is a considerable increase of 

 spark-length. 



(IV.) R^ 13 ohms, L 1= 0-00906 henry, E 1 /L 1 = 1437 r 

 2iro=6404, ^ = 2101, £=1646, 

 <£ m = 0-861, Y 2m = 34440 volts. 

 This case, when compared with the first, shows the effect of 

 changing from the series to the parallel connexion, L 2 being 

 thus reduced to one sixteenth. k x and S become very great, 

 and the effect on n is considerable. <j> m is reduced to less 

 than one-half, but since Ljj/Li is quadrupled, Y 2m is nearly 

 doubled as compared with case I. 



The maximum potentials in cases (III.) and (IV.) are 



not very different in value, the increase of the factor L 2 i/Li 



being nearly compensated by the increase of kj and 8. This 



relation depends upon the value of R i} as is shown by the 



following measurements of spark-length (Table I.). In each 



case the battery E.M.F. was 98 volts, the numbers giving 



the spark-length at make, in millimetres, between the ball 



electrodes. m x 



Table I. 



ohms. 



Spark-length in millimetres. 



Series-parallel. 



Parallel. 



10 



8-8 

 7-8 

 6-6 

 60 



143 

 9-5 

 66 

 49 



16 



25 



35 





Z2 



