26

GRAVITATIONAL METHODS

Thus, the more nearly equal the stabilizing and labilizing forces
are the more sensitive is the instrument, until the point of
instability is reached where 0 = 0' and the period becomes
infinite.

All gravimeters that are made sensitive enough to detect
changes of gravity to the order of 1 part in 10 million are also
apt to be affected by other things than gravity. Thus, most
gravimeters have a thermostat and are kept at nearly constant
temperature. They are more or less sensitive to changes in
level, to magnetic effects, and to seismic effects which may result
from very small tremors of the earth's surface. If not sealed,
they are sensitive to variations of barometric pressure.

A number of different gravimeters have been developed and
are being applied at the present time1 in routine geophysical
prospecting. For the most part, the mechanical details of
their construction and operation are not dis-
closed. However, the general operating princi-
ples of a number of instruments have been made
public. A brief description of such instruments,
classified according to whether stable or un-
stable, follows.

STABLE TYPE GRAVIMETERS

Hartley Gravimeter.—One of the earliest
descriptions of an instrument designed for com-
mercial geophysical exploration is that by
Hartley2 (Fig. 6). This instrument consists of
a mass suspended on a spring. The position of
the mass with respect to the surrounding case is
determined by an optical lever. In Hartley's
instrument the ratio of the motion of the spot of
light reflected from the optical lever to the motion of the suspended
weight was about 60,000. In practice the instrument is read
by compensating for the variation of gravity by a small auxiliary
spring which brings the suspended system back to a fixed refer-
ence position. Apparently, this instrument could detect gravity
differences of about 1 mg. It apparently has never been used

1 For a table of gravimeters in operation, see Steinmann, 1938, p. 62.
For theory and description of several gravimeters, see Heiland, 1939,

2 Hartley, 1932.

FIG. 6.—Prin-
ciple of the Hart-
ley gravimeter.