METHODS OF ATTAINING SENSITIVENESS, ETC. 15 
But to Milne we owe the application of photography to the 
problem of recording, without the introduction of any friction 
or backlash of multiplying levers. The precise method used 
by Milne will be described in the next chapter. 
The newer method of photographic registration used in the 
Bosch and Galitzin instruments depends on the principle that 
a pencil of light from a strong source of illumination may be 
reflected from a mirror attached to the pendulum and con- 
centrated at a point on the surface of the sensitive paper. An 
indicating point is thus obtained without introducing the 
slightest friction, and so the simple mathematical form of the 
fundamental equation is preserved. 
The most recent method of multiplying the motion of 
a pendulum before applying photographic registration we owe 
to Galitzin. It occurred to him that if a coil of fine wire was 
attached to the pendulum, so as to cut across the lines of a 
strong permanent magnetic field when the pendulum moved, 
electrical currents would be set up in the coil, strictly propor- 
tional to the angular velocity of the pendulum. These cur- 
rents could be carried by wires to a recording galvanometer, 
so that the movement of the needle would register photo- 
graphically on a large scale the motion of the pendulum and 
hence of the earth movement. 
The motion of the pendulum being given as before by 
6+20+70= -£/l 
then the equation for the angular motion ¢ of the galvanometer 
needle can be written in the form 
P+ 2ept+mih= —kO 
where ¢,, 7,, and £ are certain instrumental constants. There is 
thus linear relation between ¢ and x. 
This is the principle of Galitzin’s electromagnetic registra- 
tion method, where in practice both pendulum and galvano- 
meter are made to have the same period and be aperiodic 
within very narrow limits. 
