BRUNTON COMPASS ATTACHMENT FOR MEASUREMENT 
OF HORIZONTAL MAGNETIC INTENSITY’ 
JOHN H. WILSON? 
Golden, Colorado 
ABSTRACT 
Utilizing the Brunton compass in common use by geologists, an attachment has 
been devised making it possible to measure the horizontal intensity of the earth’s field. 
The operation of the instrument, the theory and derivation of equations, and the results 
of several surveys with the instrument are presented. 
INTRODUCTION 
As the Brunton compass is in almost universal use by geologists 
and engineers, an attachment which enlarges the usefulness of the com- 
pass is important. The attachment described herein is designed to meas- 
ure the absolute horizontal intensity of the earth’s magnetic field with 
sufficient accuracy to be of aid in some classes of geophysical work. 
The instrument is not intended for close or accurate magnetic investiga- 
tions, but should be used where the anomalies exceed 250 gammas. The 
accuracy of the instrument in magnetic investigations is comparable 
with the accuracy obtained by the Brunton in traverse work. 
CONSTRUCTION OF INSTRUMENT 
The attachment consists of the compass holder and an auxiliary 
magnet arm. 
The compass holder is constructed of non-magnetic metal, with 
slots which fit corresponding projections on the bottom of the Brunton 
compass. Two clamping screws prevent the compass from moving or 
falling off the compass holder when the attachment is in use. 
The auxiliary and detachable magnet arm is graduated in millimeters 
and carries a magnet holder with a vernier reading device. The magnet 
holder is moved along the auxiliary arm by means of an adjustable rack 
and pinion. The auxiliary magnet arm makes an angle of 120° (measured 
in a clockwise direction) from the north index of the compass box. In 
tRead before the Association at the San Antonio meeting, March 21, 1931. Man- 
uscript received, March 14, 1931. 
2Consulting geologist and geophysicist, 1106 Cheyenne Street. 
1391 
109 
