26 MODERN SEISMOLOGY 
constant for each instrument and is about 120 mm. It may 
be determined by observing the static sensitiveness when has 
some known value. 
Another method used by Galitzin depends on the principle 
that 2*=¢2//, so that by changing z by known amounts and 
determining the corresponding values of z we get data for 
calculating Z. 
Thus 7= g(2, - 2,)/(#,? — 19”). 
The artificial damping is cut out by removing the magnets, 
and observation of the apparent period and damping ratio 
(now nearly unity) is made, so that the values of z can be com- 
puted. The changes of z are determined by observation of a 
mirror attached to the frame, by means of telescope and scale. 
The values of e and z do not remain quite constant, so 
that it becomes important to determine them at any time zz 
salu. 
The differences of ¢ and z from their theoretical values are 
small, and are determined by observations on the recording 
galvanometer, the theory of which we have yet to consider. 
The galvanometer is of the moving coil type in which the 
suspended system is controlled by the torsion of a fine wire. 
When a current I exists in the coil a couple strictly propor- 
tional to I arises, due to the strong magnetic field in which 
the coil turns. If we neglect the self-induction of the circuit 
the current |= E/R where E is the electromotive force in the 
circuit and R the resistance. When connected to the coil in 
the pendulum E consists of two parts: (1) due to the pendulum 
motion and proportional to 6, and (2) due to the motion of the 
galvanometer coil and proportional to de 
Thus we see that the equation of motion of the galvano- 
meter coil is given by 
Ue 3g 
where e, =¢, +¢/R. 
When on open circuit (R = 0 ) the free motion of the coil is 
given by 
b+ 20h + 2,2 =0 
so that ¢, arises from a small damping action to which the 
system is naturally subject. 
