1867.] llv. C. E. Varley on Magneto-electric Machines. 403 



III. " On certain Points in the Theory of the Magneto-electric Ma- 

 chines of Wilde^Wheatstone, and Siemens." By C. F. Varley, 

 Esq. In a Letter to Professor Stokes, Sec. E.S. Received 

 Eebruary 26, 1867. 



Fleetwood House, Beckenham, S.E., 

 February 23, 1867. 



My dear Sir, — Professor Wheatstone showed that a shunt put into 

 the circuit of the electromagnet increased the power greatly, but the expla- 

 nation that it increased the power by equalizing the resistance of the arma- 

 ture and that of the electromagnet is either wholly incorrect, or very 

 nearly so. 



Yesterday I had an opportunity afforded me by Mr. C. Siemens of expe- 

 rimenting with his machine, in which the electromagnets have each a resist- 

 ance of about 250 Ohms = 500 Ohms, the armature 400 Ohms. 



On adding a shunt to the electromagnet the flame was greatly increased. 



The two electromagnets when connected in series had a resistance of 

 about 500 Ohms. I then connected them in a double circuit, the resist- 

 ance in this case being about 125 Ohms. By this means the same result 

 as regards resistance could be obtained as by a shunt, with the dilFerence 

 that the power expended in the shunt is lost in heat ; while reducing the 

 resistance by the double circuit caused the whole force to be expended on 

 the electromagnet. 



The results of the experiment, were — 



1st. The shunt invariably increased the power. 



2ndly. When the magnets were joined in double circuit the power was 

 greatly reduced. 



The explanation is to me obvious. In a Kuhmkorff's coil, where the 

 iron core is divided into fine wire, so that the dying magnetism cannot set 

 up currents in the iron core to prolong its existence, the magnetism is very 

 rapidly lost, and the make-and-break hammer works very rapidly, some- 

 times as fast as sixty beats per second. 



. If the secondary circuit be closed so that the currents can flow, the make- 

 and-break hammer works very slowly, indeed one or two beats per second ; 

 and in 1856 I published a description of electromagnets whose action was 

 very slow, and which were rendered sluggish by a copper cylinder around 

 them. 



Wilde's armature, when revolving, sends intermittent currents around 

 the electromagnet, whose circuit is broken at every half revolution of the 

 armature. 



Were the magnets composed of fine iron wire, the magnetism would die 

 away rapidly, producing a violent current b}* its efforts to maintain itself, 

 as in the RuhmkorfF's coils. (This current is called by foreigners the 

 extra-current.) 



The shunt which Wheatstone inserted carries this current across, and so 

 maintains the magnetism of the electromagnets until the armature gives a 



