142 E H. Hall— Effect of Magnetic Force on the 



were not intended to be subject to such induction, a complete 

 determination with any particular strength of magnetic field 

 required two sets of observations, one made with the cross 

 facing east (the axis of the magnet being nearly east and 

 west), the other made with the cross facing west, all elec- 

 trical connections being the same throughout the two sets and 

 the condition of the ballistic and the astatic galvanometers re- 

 maining, as nearly as practicable, unchanged. 



When magnet currents of moderate strength, e. g., 7 or 8 

 Amperes, were used, two complementary sets were usually 

 made with an interval of not more than 5 or 10 minutes be- 

 tween them, and in such cases the strength of the field was 

 not measured by means of the test- coil until the observations 

 with the cross were finished. When the strongest currents 

 were used the magnet became heated rapidly, and thermo- 

 electric effects in the astatic galvanometer circuit soon began 

 to show. In these cases three observations of the astatic gal- 

 vanometer, right, left, right, or left, right, left, as the case 

 might be, the readings being 1% minutes apart, were all that 

 could profitably be made on the transverse current. The 

 strength of field was then taken with the test- coil, but by the 

 time this had been done, the magnet had become so hot that 

 the complementary set of observations with the cross faced 

 about could not be made until several hours had passed. 

 When the second set was made the strength of the field was 

 measured again. The magnet current was closely watched 

 by means of an ammeter. This made possible certain correc- 

 tions and reductions required by the fact that this current and 

 hence the magnetic field could not be kept perfectly constant. 

 It appeared that when the currents were strongest it required 

 about 4 per cent increase in current to produce an increase of 

 1 per cent in the strength of field. A field of 8000 was in- 

 creased about 0*6 per cent by an increase of 1 per cent in the 

 strength of the current. A Gramme dynamo supplied the 

 current. 



The temperature of the crosses during the experiments is a 

 matter of some doubt. The poles of the magnet have been 

 about 7 mrn apart in all observations with crosses 5 and 7, 

 since June 1, 1888. JS r o. 7, with the glass upon which it is 

 mounted and the wax in which it is imbedded, makes a mass 

 which nearly fills the space between the poles. JSTo. 5, with 

 its mounting, is considerably thinner. Neither cross could 

 possibly come into actual contact with the poles. A few turns 

 of coarse thread were wound around the plates to prevent or 

 impair contact between the glass and the pole and so to im- 

 pede the flow of heat from pole to cross. The crosses, even 

 with the electric current which each was transmitting, prob- 



