251 



When a pliotugrapli of the- discharge parallel with the lines of force 

 was desired, the magnet was tnrned with its axis perpendicular to the axis 

 of the tube, and the glass tubes, held in position by corlj:s in the hollow 

 cores, provided insulation for the rods holding the points. In this case 

 also an auxiliary tube 12.5 cm. square and 1 meter long w^as placed between 

 the coils and telescoped into the two portions of tlie longer tube which 

 were too large to fit between the coils. Tliis small tube had a circular hole 

 in each of two sides to receive the pole pieces of the magnet, and another 

 in the upper side similar to the one in the first auxiliary tube described. 



In all cases the magnet w^as electrically connected to earth and the 

 wires bearing the current were separated from the walls of the room 

 and from the camera by means of glass tubing, when they were too near 

 for the air to insulate them. All metal parts used in the magnetic field, 

 such as screws in the auxiliary tubes, were of brass. 



When studying the deflection of the discharge due to electrostatic 

 deflection, the tube was used as shown in Fig. 1. Tw-o brass plates 

 8 by 5 cm. were placed one above the other below the points. They were 

 held in position by brass rods soldered perpendicularly to them at the 

 center. The rods were firmly fitted into glass tubes which passed through 

 the upper and lower sides of the tube. For part of the work the plates 

 were connected electrically in multiple circuit wdth the points, while for 

 the other part they were charged by means of a small Holtz machine. 

 The points were charged by a four-mica plate Wagner electrostatic machine, 

 from which the Lej'den jars had been removed. Both the Wagner and the 

 Holtz machines were run by electric motors with rheostats in circuit for 

 varying the speed. Sixteen different speeds • were . possible with the 

 Wagner, and eight with the Iloltz machine. 



The points were made of brass pins 1.15 mm. in diameter and 4 cm. 

 long. They were put in a lathe, shari)ly pointed by means of a carborundum 

 stone, and made to slope 2.5 cm. from the end. They were soldered into 

 the ends of brass rods 5.57 mm. in diameter. 



TPvANS\ERSE MACJNETIC FIELU. 



The apparatus was first adjusted with the points at right angles to 

 the direction of the magnetic lines of fdvce and the photographs of series 

 A, B, C and D were taken. 



