122 Whitehead — Magnetic Effect of Electric Displacement. 



distance was varied, and 1'9™^ was found to be the minimum 

 practicable separation, anything less resulting in the dielectrics 

 receiving a charge which would draw it against the surface of 

 one of the rings ; for any distance between the electrodes this 

 always happened in damp weather. The coil w^as carefully 

 sheathed with tin foil whicli w^as connected to earth. The 

 suspension hook on the beam was lengthened so as to reach 

 into a glass tube shown in the sketch at M and the mirror 

 placed on it there. By moving the source of light and scale 

 in a circle about the apparatus, it was thus possible to take 

 observations in any position of the beam. The walls of the 

 glass tube scattered the light somewhat, but the intensity of 

 the Nernst filament was so great as to always give a well de- 

 fined line on the scale. The whole deflecting apparatus was 

 suitably enclosed so as to be free from disturbance by air 

 drafts. 



Calculation of Expected Effect. — With 8800 volts on the 

 electrodes and with dielectrics of glass, parafiin and rock salt, 

 the order of magnitude of the displacement current is not 

 different from that already calculated. The angle of lag of 

 the current in the coil, that is, the phase of the magnetic field, 

 and also the intensity of the field, had also to be determined. 

 The angle of lag was obtained by measuring the current taken 

 by the coil at a known voltage and frequency. At 400 volts 

 and 133 cycles this current was 10 amperes, the resistance 

 being 4 ohms : 



400 , ^ , 



10 = — — , whence L = -05 henry ; 



\/l6 + 835L' 



also the angle of lag, 6, is given by 



tan 6 = ^'^ = 10-45, or 6 = 84° 28'. 



The intensity of the magnetic field at various positions within 

 the coil was determined by sending a current through it and 

 measuring the E.M.F. in an exploring coil of known area and 

 number of turns. At 200 volts the current in the coil was 5*3 

 amperes. The exploring coil consisted of 400 turns of No. 33 

 wire, and had an effective area of about 25 '8^"^. Moving the 

 exploring coil along a radius by equal steps, the following read- 

 ings give the curve, shown in fig. 5, of variation of the inten- 

 sity of the magnetic field with the distance from the coil ; 

 ordinates are values of the intensity and abscissae distances 

 from the center, the coil having a radius of 13*6'^™. The de- 

 flecting beam was 22*2'^'^ ^ou^, so that the dielectrics swing at 

 a radius of ll'l""", and at this distance the field, with 5*3 am- 

 peres in the coil, had an intensity of approximately 220, or 42 



