A NEW INSTRUMENT FOR MEASURING VERY 
SMALL DIFFERENCES IN GRAVITY 
By Kenneto Hartiey 
Hartuey Gravity BALANCE CoRPORATION, Houston, TEXaAs 
(Received January 2, 1932) 
ABSTRACT 
Description of a new portable instrument for measuring relative values of gravity 
to within two or three parts in ten million, specially designed for geophysical explora- 
tion. The principles of the design are analysed and the methods for eliminating effects 
of elastic hysteresis, temperature changes, variations in barometric pressure, etc., are 
discussed. Also effects of initial stresses in materials, defects in alignment of locking 
mechanism, inaccurate leveling, etc. These difficulties are serious but seem to have 
been overcome. Results of preliminary field measurements near Houston, Texas, are 
presented and some comparison between the type of information given by this instru- 
ment and by the torsion balance. 
NTIL now the only successful method of measuring gravity has been 
with some form of pendulum, in other words determining the accelera- 
tion produced, or, we may say, obtaining the value of gravity in terms of 
inertia. There is only one other possible method and that is to measure it in 
terms of the elasticity of a spring which can be calibrated in some way. This 
has the advantage of eliminating time measurements but introduces some 
other difficulties that are avoided by the pendulum. 
In all previous attempts that I know of to use a spring the effort has been 
to obtain extreme sensitivity by some arrangement approaching unstable 
equilibrium, and one reason for their failure is that this unstable condition 
also exaggerates all the causes of error and uncertainty in the indications of 
the apparatus. In the new instrument the effort has been in the other direc- 
tion, that is to obtain the maximum stability, the small displacements are 
then amplified by optical means with less uncertainty. 
The fundamental idea of the design is that if we can apply to a suspended 
mass an upward force almost, but not quite, equal to the pull of gravity and 
maintain this force constant then it will be easy to measure changes in the 
small additional force required to hold the system in equilibrium. The first 
problem then is to apply a force that can be maintained constant to the re- 
quired degree of accuracy. The value of such an instrument for geophysical 
prospecting depends on its ability to measure very minute differences in the 
gravitational force, so the instrument has been designed to have the greatest 
sensitivity that can be of any practical use. Since the change in the gravita- 
tional attraction due to a difference of one foot in elevation is almost exactly 
one part in ten million this was taken as the maximum sensitivity that could 
possibly be useful; for most purposes three or four parts in ten million will be 
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