THE MAGNETO-ELECTRIC CURRENT. 103 



Dr. Joule in 1843, ^^ ^^e result of a delicate investigation 

 on tlie quantitative relation existing between ordinary- 

 mechanical power and heat"^. In the electro-magnetic 

 machines of my invention this phenomenon unfortunately 

 manifests itself on an alarming scale_, so much so that the 

 armature of the lO-inch machine rises in the course of a 

 few hours to 300° F. and upwards ; and were the action 

 of the machine to be continued for any lengthened 

 periodj the insulation of the armature- coils would be en- 

 dangered. 



One method of mitigating this evil was to construct the 

 machine of smaller dimensions, so as to afford greater 

 facilities for the dissipation of the heat by radiation and 

 conduction. But even in the smaller machines an incon- 

 venient residuum of heat still remained when they were 

 worked continuously for a considerable time, so as to 

 render it desirable to adopt some means for abstracting 

 the heat more rapidly. By means of a current of water 

 circulating in the hollow brass segments which form part 

 of the magnet-cylinder, Mr. Charles E. Eyder, the skilful 

 manager at the works of Messrs. Elkington and Co., has 

 happily succeeded in so far reducing this heating as to 

 permit of the machines being worked for days and nights 

 together without intermission, and without any sensible 

 diminution of the power of the current. 



The machines which have been found to be the most 

 efficient and economical in their working are those which 

 have armatures from 3I to 4 inches in diameter. The ar- 

 matures are driven at about 2000 revolutions per minute ; 

 and the water, after having passed through the magnet- 

 cylinder, is used for supplying the boilers which furnish 

 the power for driving the machines. 



I have already shown elsewhere that the current from a 



* Phil. Mag. S. 3. vol. xxiii. p. 264. 



