28 INDUCTION SHOCKS 
interior of the glass tube is clear of liquid. The elec- 
trode is shown in Fig. 6; contact is made by rotating 
slightly the stopper carrying the two platinum wires. 
For the determination of " specific " stimulation val- 
ues (see p. 76), a rather large known resistance, ten 
thousand to twenty thousand ohms, must be arranged to 
be included in the secondary circuit as required. 
FIG. 6. Shielded electrodes (Sherrington). 
The arrangement of apparatus for making the cali- 
bration is illustrated diagrammatically in Fig. 7. The 
procedure is by the following steps, for each of which 
a page reference is given. 
1. Determination of the formula for core magnetiza- 
tion (p. 43). 
2. Determination of the mutual induction for a series 
of selected secondary positions from 12 cm. outward 
(p. 38). 
