[ 543 ] 

 LXIX. Proceedings of Learned Societies. 



ROYAL SOCIETY. 



[Continued from p. 4/8.] 



March 14, 186/. — Lieut. -General Sabine, President, in the Chair. 



rPHE following communications were read : — 

 -*- " 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. R.S. 



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 explanation that it increased the power by equalizing the resist- 

 ance of the armature 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 

 experimenting with his machine, in which the electromagnets have 

 each a resistance of about 250 Ohms == 500 Ohms, the armature 400 

 Ohms. 



On adding a shunt to the electromagnet the flame was greatly in- 

 creased. 



The two electromagnets when connected in series had a resist- 

 ance of about 500 Ohms. I then connected them in a double cir- 

 cuit, the resistance in this case being about 1 25 Ohms. By this means 

 the same result as regards resistance could be obtained as by a shunt, 

 with the difference 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 HuhmkorrT'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, sometimes 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 re- 

 volution of the armature. 



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

 die away rapidly, producing a violent current by its efforts to main" 



