114 Whitehead — Magnetic Effect of Electric Displace^nent. 



rigidly at each end of a light beam which was suspended hori- 

 zontally on a quartz fiber attached at its center. The phase of 

 either the electric or the magnetic field on one block being 180° 

 from that on the other, the other field Laving the same phase 

 for each block, the reaction of the displacement current on the 

 magnetic field would be opposite at the two ends of the beam, 

 causing a couple to act on the fiber suspension. 



Apparatus. 



First form, — The first form of the apparatus is shown in fig. 

 1, and i^^. 2 gives a central section. AA are the rectangular 

 blocks of dielectric, each attached to the beam D, which was 

 constructed of bamboo or glass : B B B B ai-e brass electrodes 

 each turned so that its surface is a part of a circular cylinder 

 with its axis coincident with the line of suspension of the beam ; 

 this was thought to give the best approximation to a uniform 

 electric field between the electrodes. Each pair of electrodes 

 was connected to the terminals of a transformer giving 8800 

 volts at 133 cycles per sec. C C are circular coils of wire, one 

 surrounding each pair of electrodes, the planes of the turns 

 being horizontal ; they are supplied with alternating current 

 from the same generator which excites the electrodes. The 

 magnetic field is seen to be vertical, the electric field horizontal, 

 so the resulting deflection of the beam should be out of the 

 plane of the paper at one end and in at the other. The whole 

 rested on a wooden base, and each half of the apparatus was 

 enclosed in a brass case, there being between the two only a 

 small connecting trough in which the beam could swing, thus 

 making the interior as small as possible and so minimizing the 

 disturbance due to air currents. A close fitting brass cylinder 

 inside the coil, a ring on top, and tin foil on the fioor, all con- 

 nected to the case, which was grounded, protected the dielectric 

 from any electric field due to the coil ; all possible secondary 

 circuits were split, the openings being closed with hard rubber 

 or fibre. The damping vane shown in the sketch was a strip 

 of mica immersed in water, attached to the deflecting beam by 

 means of a fine glass rod. The quartz fiber used was approxi- 

 mately 102'^°' (40^^) long in all cases; it was enclosed in a 

 glass tube fitted with a convenient torsion head permitting the 

 adjustments of torsion and length independently ; the fibre 

 carried a small copper hook at its lower end which fitted into 

 a corresponding hook sealed to the glass beam. Deflections 

 of the beam were observed by means of the movements on a 

 ground glass scale of the image of an incandescent lamp fila- 

 ment reflected froin a small mirror on the center of the beam ; 

 during the later experiments the lamp was replaced by a IS^ernst 



