472 Royal Society : — Mr. Wheatstone on the Augmentation 



galvanometer, by its heating 4 inches of platinum wire '0067 in dia- 

 meter, by its making a powerful electromagnet, by its decomposing 

 water, and by other tests. 



The explanation of these effects is as follows : — The electro- 

 magnet always retains a slight residual magnetism, and is therefore 

 in the condition of a weak permanent magnet ; the motion of the ar- 

 mature occasions feeble currents in alternate directions in the coils 

 thereof, which, after being reduced to the same direction, pass into 

 the coil of the electromagnet in -such manner as to increase the 

 magnetism of the iron core ; the magnet having thus received an 

 accession of strength, produces in its turn more energetic currents 

 in the coil of the armature ; and these alternate actions continue until 

 a maximum is attained, depending, on the rapidity of the motion 

 and the capacity of the electromagnet. 



If the two coils be connected in such manner that the rectified 

 current from the coil of the armature passes into the coil of the elec- 

 tromagnet in the direction which would impart a contrary mag- 

 netism to the iron core, no current is produced, and consequently 

 there is no augmentation of magnetism. 



It is easy to prove that the residual magnetism of the electro- 

 magnet is the determining cause of these powerful effects. For 

 this purpose it is sufficient to pass a current from a voltaic battery, 

 a magneto-electric machine, or any other rheomotor, into the coil of 

 the electromagnet in either direction, and it will invariably be found 

 that the direction of the current, however powerful it may eventually 

 become, is in accordance with the polarity of the magnetism im- 

 pressed on the iron core. 



If, instead of the currents in the coil of the rotating armature 

 being reduced to the same uniform direction, they retain their alterna- 

 tions, no effects, or at most very small differential ones, are produced, 

 as no accumulation of magnetism then takes place. 



I will now call attention to the fact that stronger effects are pro- 

 duced at the first moment of completing the combined circuit 

 than afterwards. The machine having been put in motion, at the 

 first moment of completing the circuit 4 inches of platinum wire were 

 made red-hot ; but immediately afterwards the glow disappeared, and 

 only about one inch of the wire could be permanently kept at a red 

 heat. This diminution of effect was accompanied by a great increase 

 of the resistance of the machine. The cause of the momentary strong 

 effect was, that the machine from its acquired momentum continued 

 its motion for a few seconds, though it required a stronger force 

 than could be applied to maintain that motion. Each time the cir- 

 cuit is broken and recompleted the same effect recurs. 



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

 into the circuit formed by connecting the coils of the electromagnet 

 and rotating 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 



