628 Dr. W. M. Thornton on the 
The curves of rise were difficult to obtain photographically, 
the spot of light moving at about 140 cms. a second. It was 
necessary to greatly overdevelop the extra-rapid bromide 
paper used. 
Fic. 4.—Rise of Current in Coil with Laminated Core. 
ee - 
aa —— — a ee eee. 
Pesca FOUN Ra — 
BaP. 
BP. 
\/ 
4 
{ 
Het 
Boise 
BESSeeRewRoe 
BeGRIBAHeeGs 
BeEDRHORREEL 
Baas ae 
BEB 
++ A ee 
rane = we 
—— = AS 
Te | |) ee 
{ 5 
“oS . : . z 
PERMEABILITY SECONDS = ba 
The general expression for the voltage in an inductive 
circuit to which a steady voltage e is applied is 
e=Prl ae (1) 
Here N is the total magnetic change and equals Li. The 
inductance L is, in all cases where iron is used, a variable. 
Hence 
di .dh 
e=rithL +i7 : ae. 
and since L is proportional to 4 we may write 
ee een) 
e=rit kya, + kes Je ee 
or be) On cH) 
ie ir hE =€... \¢) =<. 
In curve I,, fig. 4, the current pauses for some time after 
switching on, du/dt must therefore be exceedingly large since 
both 2 and di/dt are smal]. If desired it can be found, for all 
the other quantities in (4) are determinable. 
— 
