MAGNETIC REACTIONS 39 



FURTHER EXPERIMENTS WITH THE EDDY CURRENTS. 



The great intensity of the currents in the disk was also made evident 

 by the following quite elementary experiments : 



(I) Compass tests. A small pocket compass held near the disk 

 showed the presence of a strong magnetic field due to the eddy currents, 

 even at a considerable distance from the electro-magnet. One way of 

 testing this was to trace out the magnetic lines parallel to the surface 

 of the disk by the usual step-by-step method, holding the compass with 

 its plane vertical like a dip needle, close to the disk near one pole of the 

 magnet, and then advancing it by stages parallel to the disk and along 

 the direction of the lines. In fig. 16 the heavy lines marked were thus 

 obtained with the disk stationary, showing the direction of the stray lines 

 from the electro-magnet. 1 The dotted lines marked 390 were obtained 

 when the disk rotated at 390 revolutions per minute. In this figure the 

 north pole of the electro-magnet is on the side toward the observer and 

 the disk rotates counter-clockwise. Observations at points on the other 

 side of the magnet pole showed a corresponding change in the direction 

 of the resultant magnetic field when the disk was in rotation. The point 

 Q, just outside the disk, is a neutral point, where the field due to the eddy 

 currents is equal and opposite to that due to the magnet. 



(II) Galvanometer tests. The copper leads from a sensitive gal- 

 vanometer were touched to the surface of the disk at points from 1 to 5 

 mm. apart, the points being so oriented that the galvanometer showed 

 no deflection. Care was taken to reduce the effect of thermo-electric 

 forces to a minimum. This is the old method used by Faraday and Nobili 

 for plotting the lines of current flow. Though it can not always be as- 

 sumed that the current flows in a direction perpendicular to the line 

 joining these "equipotential" points, still they furnish an approximate 

 idea of the direction taken by the current paths. A few such pairs of 

 points are indicated in fig. 16, and with their aid some of the current lines 

 have been constructed, the arrow-heads indicating the direction of flow. 

 These lines must not be confused with the magnetic lines described above. 

 Tests made close to the magnet pole proved that at 390 revolutions per 

 minute the inwardly directed current lines were confined to a narrow 

 band about a centimeter wide, near the trailing edge of the pole, as shown. 

 The demagnetizing effect of the currents is here very evident. 



(III) Intensity of the eddy currents. The galvanometer leads were 

 touched to the disk, as described above, at a point near the magnet pole, 

 but oriented in such a way as to produce a maximum deflection. From 

 the distance between the points of contact and the resistance and sen- 

 sitiveness of the galvanometer, the potential difference between the 

 points was found, and from this and the specific resistance of copper 



1 At the time of these tests the magnetic poles were pushed in about 2 cm. from the 

 outer edge of the disk. This can hardly have produced an appreciable change in any 

 of the quantities observed (c/. fig. 13). 



