A SUMMARY OF PROCEDURE 25 
peres and reading directly to o.i mil ampere, and to 
provide it with two shunts, one adjusted to carry nine- 
tenths of the total current, the other to carry ninety- 
nine one-hundredths of the current. For these shunts 
I use an ordinary Porter metal-contact rocking key 
connected as shown in the diagram, Fig. 5. For the 
T V shunt, German silver wire is used between one 
pair of end contacts; for the $$ shunt, copper wire is 
used between the other pair of end contacts. To cali- 
brate the shunts, resistance is introduced into the am- 
meter circuit until exactly o.oi ampere is flowing; then 
the shunts are adjusted until the ammeter reading is 
exactly o.ooi ampere, when the & shunt is in circuit, 
and o.ooo i ampere when the 1% shunt is in. The 
shunts must be recalibrated at frequent intervals, but 
this is not a difficult task. 
As a means of adjusting the amount of primary cur- 
rent flowing I have found a dial resistance box most 
satisfactory, although any available variable resistance 
can be used. The total resistance should not be less 
than 11,000 or 12,000 ohms, since with a source of cur- 
rent yielding 2 volts that amount of resistance is often 
necessary to cut the current down to the point where 
threshold stimuli are produced. 
For making and breaking the primary circuit some 
form of automatic key is required. A satisfactory one 
is described in Chapter IX. Experience shows that trust- 
