1394 JOHN H. WILSON 
7. Read the distance, 7, on the auxiliary magnet arm by means of 
the vernier to the nearest one hundredth of a centimeter. 
At the conclusion of an observation, the compass box may be closed 
and the instrument may be carried already set up to the next station. 
THEORY 
The general theory of such a set-up can be obtained from any good 
book of physics.’ 
The equation which applies to the instrument when used as above 
is as follows: 
I 
GM ee premennas 
H = 7 Pia ee (1) 
272s r6r4 
where H is the horizontal intensity at the desired point; M is the mag- 
netic moment of the auxiliary magnet; is the distance between the center 
of the needle and the center of the magnet; and L is the length of the aux- 
iliary magnet. 
If the values of 7 are large in comparison with L, equation (1) reduces 
to 
M 
jel Eo (2) 
For anomalies of fair magnitude, equation (2) is sufficiently accurate. 
As in any survey L and M are constant, a table or graph of the value 
of H for any value of r can be computed and used where absolute values 
are desired. The magnetic unit must, of course, first be determined by 
any of the customary methods. 
When effects, such as temperature and diurnal variation, are less 
than the observational errors of the instrument, they may be neglected. 
PROBLEMS TO WHICH APPLICABLE 
With the vernier arrangement it is possible to read r to the tenth of 
a millimeter. With the strength of magnet commonly used it would be 
impossible to obtain the intensity closer than 25 gammas. Repeated 
set-ups by a competent observer at the same station checked within 
0.025 centimeters of the mean value, which is equivalent to a departure 
*William Watson, Text Book of Physics (Longmans, Green and Company, 1919). 
112 
