470

INVENTIONS OF NIKOLA TESLA.

lines and arrows m and n ; and the direction of the energizingfield current being indicated by the arrows ab c d, an inspection ofthe figure shows that one of the two branches of the eddy current,that is, a b' to b, will tend to demagnetize the field, while theother branch, that is, a b' n b, will have the opposite effect.

Therefore, the branch a b' m b, that is, the one which is approach-ing the field, will repel the lines of the same, while branch a b' *

n b, that is, the one leading the field, will gather the lines offorce upon itself.

In consequence of this there will be a constant tendency toreduce the current flow in the path a b' m b, while on the otherhand no such opposition will exist in path ab'»b, and the effectof the latter branch or path will be more or less preponderatingover that of the former. The joint effect of both the assumedbranch currents might be represented by that of one single cur-rent of the same direction as that energizing the field. In otherwords, the eddy currents circulating in the disc will energize thefield magnet. This is a result quite contrary to what we mightbe led to suppose at first, for we would naturally expect that theresulting effect of the armature currents would be such as tooppose the field current, as generally occurs when a primary andsecondary conductor are placed in inductive relations to eachother. But it must be remembered that this results from thepeculiar disposition in this case, namely, two paths being affordedto the current, and the latter selecting that path which offers theleast opposition to its flow. From this we see that the eddycurrents flowing in the disc partly energize the field, and for thisreason when the field current is interrupted the currents in thedisc will continue to flow, and the field magnet will lose itsstrength with comparative slowness and may even retain a cer-tain strength as long as the rotation of the disc is continued.

The result will, of course, largely depend on the resistance ^

and geometrical dimensions of the path of the resulting eddycurrent and on the speed of rotation; these elements, namely,determine the retardation of this current and its position relativeto the field. For a certain speed there would be a maximumenergizing action; then at higher speeds, it would gradually falloff to zero and finally reverse, that is, the resultant eddy currenteffect would be to weaken the field. The reaction would bebest demonstrated experimentally by arranging the fields n s,n' s', freely movable on an axis concentric with the shaft of the