CHIBI LE YPES OF SEISMOGRAPHS IN ACTUAL USE \.%9 
The supporting wires are of steel and platinum-iridium and 
pass one from the top of the frame to a point just behind 
the mass, and the other from the inner end of the rod to the 
foot of the frame. 
At the outer end of the pendulum rod are seen the copper 
plate and damping magnets, while just behind these are seen 
the similar pair of magnets which excite the currents in the 
flat coils attached to the pendulum. These coils are connected 
to the stout leads of the galvanometer, the connexion being 
made by fine bronze strips so as not to interfere with the free 
movement of the pendulum. Both pairs of magnets are pro- 
vided with screw adjustments so that the damping and magni- 
fication may be controlled to the desired extent. Small 
mirrors are attached to the pendulum and frame, and these 
with a small electric hammer for giving the pendulum a slight 
blow are required in the process of standardizing. 
The galvanometer is of the Deprez-D’arsonval moving coil 
type. 
The registration of the movements of the galvanometer 
mirror is made photographically. The galvanometer is set at 
a convenient distance, say 1 m., from the recording cylinder, 
and the mirror is illuminated by means of collimator and slit, 
so that the reflected beam falls normally on the cylinder, while 
the image is focussed for that distance. The image is concen- 
trated to a small luminous point by the intervention of a 
cylindrical lens. The cylinder has a circumference of nearly 
I m. and revolves in about half an hour, so that the actual 
paper speed is 30 mm. per minute. The cylinder also moves 
sideways about I cm. per revolution so that the record runs 
for twelve hours. In practice we may arrange for both hori- 
zontal components, from duplicate pendulums set at right 
angles, to be recorded side by side on the same sheet. This is 
an economy of expense and possesses certain distinct advan- 
tages. But against this must be set the fact that when a large 
earthquake occurs the confusion of the record may become 
very troublesome. Thus at Pulkowa the practice is to record 
each component on a separate sheet, while two scales of regis- 
tration are used. 
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