INDUCTION SHOCKS 
slant source of current sufficient in amount to yield at 
least i ampere through the resistance of the primary 
coil. Where a charging current is available, probably a 
good storage battery will be found most convenient as a 
source of current. Several Daniell cells in series, how- 
ever, answer every pur- 
pose. A good ammeter 
for measuring the inten- 
sity of the primary current 
is required, as is also a 
variable resistance for ad- 
justing its amount. Since 
it is often necessary in 
the course of the work 
to use currents ranging 
from o.oooi ampere to i 
ampere the ammeter must 
be able to cover this range. 
No instrument is, of 
course, able to measure 
FIG. 5. Diagram showing ammeter the small currents with 
shunt made from a Porter metal- suffident accuracy and 
contact rocking key. 
at the same time to give 
direct readings for the larger ones. To give the 
ammeter the desired range, therefore, recourse must be 
had to a system of shunts. I have found it convenient to 
use a milammeter having a scale capacity of 10 mil am- 
