174 OH AGIs 
circuit must be maintained in a much better condition than is required 
in the more common electrical measurements or else some provision 
must be made in the design to obviate the effects of leakage currents. 
In the apparatus developed at the Department of Terrestrial 
Magnetism by the writer, the contact potentials, etc., are eliminated 
by employing a double commutator which reverses the energizing 
current about twenty times per second and at the same time rectifies 
the pick-up current so that a direct-current potentiometer may be 
used in this part of the measurement. If this is to be satisfactorily 
accomplished, the commutator must be carefully made and properly 
adjusted. When the contact potentials, for example, are very large, 
0.5 volt and upwards, the forced vibration of the pointer of the 
potentiometer-galvanometer becomes so great that settings are diffi- 
cult. This difficulty is eliminated if a condenser of about 20 micro- 
farads capacitance is thrown in series with one of the pick-up lines. 
The condenser should not be required when the contact potentials 
are much less than 0.5 volt unless the galvanometer has characteris- 
tics which make it unsuitable for this work or the adjustment or con- 
struction of the commutator is defective. If the latter be the cause, 
serious systematic errors may enter the measurements. 
The effects of induction between the two circuits can be very seri- 
ous, especially when measurements are being made with long lines. 
Some workers have attempted to correct for the effects by obtaining 
measurements at two different and known rates of commutation, but 
in the opinion of the writer this is not satisfactory either in theory or 
in practice. These induction effects may, however, be avoided if the 
potential between the pick-up points is measured only during that 
part of the cycle when the energizing current is constant. That this 
can be accomplished automatically if the commutator is designed and 
adjusted with this end in view has been demonstrated by many com- 
parisons made by W. J. Rooney.! It is, of course, obvious that if the 
commutator in the present form should be used with lines many times 
the length of any employed heretofore in surveys, the time constant 
of the circuit may become so’ great that adjustment would no longer 
be possible. It should also be mentioned that an improperly adjusted 
commutator can falsify the potential measurements by effectively 
shunting the potentiometer during a part of the cycle. 
Errors from defective insulation may at times be very great. Dur- 
ing early experience in this work, errors of several hundred per cent 
‘ Terr. Mag., 35 (1930), pp. 61-72 
504 
