194 HENRY A. EOWLAND 



measure, an earth inductor was included in the circuit with the little 

 coil and galvanometer and the deflections produced by this were com- 

 pared with those produced by moving the little coil. These deflections 

 were taken between every two observations with the little coil. 



The deflections due to moving the little coil, those due to the earth 

 inductor and that due to pulling the coil away from the centre are 

 given in the following table: 



Distance a. Distance 6. Distance c. 



Coil 4-407 cm. 9-655 cm. 6-363 cm. 



Earth inductor 33-138 cm. 33-137 cm. 33-162 cm. 



Drawing coil away from centre 57-416 cm. 



In order to determine the proper quantities for substitution in the 

 expression for the magnetic potential of the field, it was necessary to 

 measure, besides, the deflections due to the little coil when moved 

 through various distances and those due to the earth inductor. 



The mean radius of the small coil = -3912 cm. 



Number of turns = 83 



Width if coil = -182.4 cm. 



Depth of coil = -1212 cm. 



Integral area of earth inductor = 20716-2 cm. 



Horizontal intensity of earth's magnetism. . . . = -1984cgs. 



The quotient of the mean radius of the coil by the distance moved 

 gave tan d. 



The linear measurements were made with a dividing engine. 



The horizontal intensity of the earth's magnetism was determined 

 by measuring the time of swing of a bar magnet and its effect upon a 

 smaller galvanometer needle. The proper substitution of these quan- 

 tities in the formula given gave the expression in absolute measure 

 for the magnetic potential at any part of the field. 



The remaining part of the experiment and the part that was attended 

 with greatest difficulty, was to prepare little bars of the substances and 

 to determine the times of vibration of these when suspended, first with 

 the axis vertical and then with it horizontal in the magnetic field. 

 Besides this, the dimensions and the moment of inertia of each bar had 

 to be determined, and, in the case of the calc-spar, the angle the bar 

 made with the equatorial line of the poles when in its position of equi- 

 librium, had to be measured. 



Bismuth and calc-spar were the two crystals experimented upon; 

 quite a number of other substances were tried but failed to give good 



