1867.] 



of the Power of a Magnet hy reaction. 



371 



On bringing the primary coil of an inductorium (Ruhmkorff's coil) into 

 the circuit formed by connecting the coils of the electro-magnet and rota- 

 ting armature, no spark occurs in the secondary coil. On account of the 

 great resistance of the circuit, which now also includes the primary coil of 

 the inductorium, the current is not in sufficient quantity to produce any 

 noticeable inductive effect. 



A very remarkable increase of all the effects, accompanied by a diminu- 

 tion in the resistance of the machine, is observed when a cross wire is 

 placed so as to divert a great portion of the current from the electro- 

 magnet. The four inches of platinum wire, instead of flashing into redness 

 and then disappearing, remains permanently ignited. The inductorium, 

 which before gave no spark, now gave one a quarter of an inch in length ; 

 water was more abundantly decomposed ; and all the other effects were 

 similarly increased. 



I account for this augmentation of the effects in the following way : — ■ 

 Though so much of the current is diverted from the electro-magnet by 

 the cross wire, the magnetic effect still continues to accumulate, though 

 not to so high a degree ; but the current generated by the armature, passing 

 through the short circuit formed by the armature-branch and cross wire, 

 experiences a far less resistance than if it had passed through the armature 

 and electro-magnet branches ; and though the electromotive force is less, 

 the resistance having been rendered less in a much greater proportion, the 

 resultant effect is greater. 



I must observe that a certain amount of resistance in the cross wire is 

 necessary to produce the maximum effect. If the resistance be too small, 

 the electro-magnet does not acquire sufficient magnetism ; and if it be too 

 great, though the magnetism becomes stronger, the increase of resistance 

 more than counterbalances its effect. 



But the effects already described are far inferior to those obtained by 

 causing them to take place in the cross wire itself. With the same appli- 

 cation of force, 7 inches of platinum wire were made red-hot, and sparks were 

 elicited in the inductorium 2\ inches in length. 



The force of two men was employed in these, as well as in the other ex- 

 periments. When the interrupter of the primary coil was fixed, the ma- 

 chine was much easier to move than when it acted. For when the inter- 

 rupter acted, at each moment of interruption the cross wire being, as it 

 were, removed, the whole of the current passed through the electro-magnet, 

 and consequently a greater amount of magnetic energy was excited, while 

 in the intervals during which the cross wire was complete the current 

 passed mainly through the primary coil. 



The effects are much less influenced by a resistance in the ebctro-mag- 

 net branch than in either of the other branches. 



To reduce the length of the spark in the inductorium (the primary coil 

 of which was placed in the cross wire) to | of an inch, it required the re- 



VOL. XV. 2 H 



