of the Power of a Magnet by reaction, 473 



diminution 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 electromagnet. 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 electromag- 

 net by the cross wire, the magnetic effect still continues to accu- 

 mulate, though not to so high a degree ; but the current generated by 

 the armature, passing through the short circuit formed by the arma- 

 ture-branch and cross wire, experiences a far less resistance than if it 

 had passed through the armature and electromagnet 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 electromagnet does not acquire sufficient magne- 

 tism ; 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 

 application 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 experiments. When the interrupter of the primary coil was 

 fixed, the machine was much easier to move than when it acted. 

 For when the interrupter acted, at each moment of interruption the 

 cross wire being, as it were, removed, the whole of the current passed 

 through the electromagnet, 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 electro- 

 magnet 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 f of an inch, it required 

 the resistance of 5^- inches of the fine platinum wire in the cross wire, 

 5 inches in the armature-branch, and 4 feet in the electro-magnet 

 branch. 



When there was no extra resistance in either of the branches, the 

 length of the cross wire being only about a few feet, the intensity of 

 the current in the electromagnet branch, compared with that in the 

 cross wire, was as 1 : 60 ; and when the resistance of the primary 

 coil of the inductorium was interposed in the cross wire, the relative 

 intensities were as 1 : 42. 



In conclusion I will mention that there is an evident analogy 



