414 PROCEEDINGS OF THE AMERICAN ACADEMY 



XXIX. 



CONTRIBUTIONS FROM THE PHYSICAL LABORATORY OF 

 HARVARD COLLEGE. 



No. XIIL — ON THE INTENSITY OF TERRESTRIAL 

 MAGNETISM AT CAMBRIDGE. 



By Henry Goldmark. 



Presented April 10, 1878. 



As the intensity of the force of the earth's magnetism has not been 

 determined in Cambridge for many years, it was thought that a meas- 

 urement of its magnitude miglit 23rove of some interest. I measured 

 only the horizontal component and the inclination, and deduced the 

 value of the vertical component from these. 



To obtain the horizontal component, I made use of Gauss's method 

 of oscillations, using the torsion balance made for this purpose by 

 Edelmann, of Munich. Two quantities are determined, the product 

 MH cii the horizontal intensity H and the magnetic moment J/ of the 



magnet used, and the ratio — of these two quantities. 



To get MH, the time of oscillation of a small cylindrical magnet, 

 suspended by a silk thread, was determined by means of a mirror and 

 scale. The number of complete oscillations and fractions of an oscil- 

 lation which this magnet made in one minute of time when vibrating 

 under the influence of the magnetic force was observed, and from this 

 the time < of a single oscillation was easily obtained. The amplitude 

 of vibration was in every case so small that the usual reduction to an 

 infinitely small arc was found to be unnecessary. 



The coefficient of torsion O of the thread was in every case found 

 by turning the upper circle through an angle of 90°, observing the 

 angular deflection (gi) from the magnetic meridian produced in the 

 suspended magnet, and substituting this value in the equation 



90° — ^ 



