242 MR J. C. BEATTIE ON THE 



current; the poles were circular surfaces 60 mm. in diameter and 18 mm. apart. By- 

 inserting suitable resistances in the electro-magnet circuit any field required could be 

 obtained. 



The field strength was measured by Verdet's method : as the strength does not come 

 directly into the calculations it is given only approximately. The necessary measurements 

 were made some weeks after the other experiments. 



The plates used were fixed on to strips of ebonite ; at both ends copper of the same 

 breadth and thickness was soldered on, the ends of the copper dipped into pools of 

 mercury. The two pools could be connected with the primary current and with the 

 galvanometer simultaneously ; in this way the resistance of the plate perpendicular to the 

 direction of the field was measured. It is to be noted that the resistance of the copper 

 plates comes in, but as this does not vary in a magnetic field, An is not affected. 



To measure the transverse effect and the resistance along the lines of force two wires 

 arranged as in fig. 1, were soldered on to the two middle points of the sides of the 

 bismuth plate ; the ends of these wires dipped into four small mercury pools. 



The plates so arranged could be clamped in the field in either of two positions at 

 right angles to one another. 



Three different positions of the plate with respect to the lines of force of the field 

 were considered. 



Suppose the direction of the field to be parallel to the plane of the paper, and let this 

 be our y-axis : let the 2-axis be drawn perpendicularly upwards, the ai-axis towards the 

 reader. In the first position (a) the plate's surface was in the xz plane, and the primary 

 current flowed in the z direction. In the second position (/3) the plate's surface was in 

 the yz plane, and the primary current flowed in the direction z. The resistance 

 measured in both these cases is the resistance perpendicular to the lines of force of the 

 magnetic field. 



In the third position (y) the plate's surface was in the yz plane, and the primary 

 current flowed in the direction y. With this arrangement the resistance along the lines 

 of force could be measured by sending the primary current in at (1) or (2), while at the 

 same time (3) and (4) were joined to the galvanometer. 



It was found, however, that this latter arrangement was not very suitable, and in the 

 greater number of cases another method (fig. 8) was used. The plate was fixed on to 

 another piece of ebonite. Along the sides thick copper wires were soldered throughout the 

 whole length ; these served for the primary current. Two other wires were soldered along 

 the length of the plate, but were not in direct contact with the other two : one end of 

 each of these was joined to the galvanometer. 



The transverse effect was measured with the plate in position a. 



No attempt was made to keep the temperature of the plate constant by using liquids ; 

 the temperatures given are the approximate temperatures of the room during the time of 

 the experiment. Between the different experiments, however, a pause was made to allow 

 the plate to cool. 



