26 INDUCTION SHOCKS 
worthy results cannot be obtained with a key which 
fails to give uniform breaks. Uniform makes are very 
desirable, but for many sorts of work, including the 
routine of making the calibration, make shocks need not 
be employed. 
All the apparatus thus far described is required for 
the quantitative use of the induction coil as well as for 
its calibration. Additional instruments needed for mak- 
ing the calibration are a good ballistic galvanometer and 
a standard induction apparatus. A satisfactory form 
of ballistic galvanometer is the d'Arsonval wall instru- 
ment with moving coil and reflected scale, read with a 
telescope. The standard induction apparatus can be 
made in any machine shop. It consists of a primary 
coil, at least 75 cm. long and composed of a single layer 
of heavy insulated wire, carefully wound, and a sec- 
ondary coil, not over 15 cm. long, of about 2000 turns 
of fine wire, placed exactly at the center of the primary 
coil. The cross section and number of turns per centi- 
meter of the primary coil must be known, and the total 
number of turns of the secondary. 
Additional apparatus required in the use of the in- 
ductorium, but not in the calibration, is, first, a device 
for determining tissue resistance, and, second, suitable 
stimulating electrodes. I have found the Kohlrausch 
method of measuring resistance perfectly satisfactory 
(see p. 72). This method requires an ordinary meter 
