1880.] On Magnetic Circuits in Electric Machines. 



287 



VL " On Magnetic Circuits in Dynamo- and Magneto-Electric 

 Machines. No. 2." By Lord Elphinstone and Charles 

 W. Vincent, F.R.S.E., F.C.S., F.I.C. Communicated by 

 Professor G. G. Stokes, Sec. R.S. Received March 10, 

 1880. 



A large amount of magnetism is retained by the soft iron cores of 

 electro-magnets, when arranged so as to form a complete magnetic 

 circuit : and sparks and other indications of the passage of an 

 electric current can be obtained at the ends of the helix wires sur- 

 rounding those soft iron cores, each time the masses of iron are 

 separated and the closed magnetic circuit opened. In order to pro- 

 cure a spark the breaking of the circuit must be effected suddenly, 

 either by a jerk, tilt, or sliding movement. 



In the case of the 58 lb. magnet described in our former note, the 

 current that is capable of causing a spark, although only momentary 

 in duration, is found to be sufficient in quantity and intensity to 

 magnetize a small electro-magnet, weighing with its coils between 

 5 and 6 lbs., enabling it to sustain its own weight for any indefinite 

 time when suspended by its armature. 



When the armature of the small magnet is placed at the distance 

 of §• of an inch from its poles, in such a manner as to be free to move, 

 the instant the armature of the large magnet is suddenly tilted or slid 

 off it darts to them, the completion of the circuit of the small 

 magnet being signalled by a smart click. The rupture of one closed 

 magnetic circuit is thus caused to produce another closed magnetic 

 circuit. 



But when the interval between armature and magnet, whose circuit 

 it was intended to close, exceeded \ of an inch, the former was not 

 attracted with sufficient force to overcome the friction of the table 

 upon which it was resting. 



The mode of removing the armature from the large magnet 

 appeared to be of no moment, but the time occupied by the removal 

 had much influence upon the amount of magnetic force manifested in 

 the smaller circuit. This was particularly the case if there were an 

 interval, no matter how small, between the armature and the poles of 

 the magnet round which the electric current was sent. 



For example, if with an interval of of an inch between the 

 armature and the poles of the small magnet, the armature of the 

 large magnet was slowly slid off, the magnetization of the small 

 magnet never rose to a sufficient intensity to draw its keeper to itself, 

 whereas, when the sliding took place rapidly, the small armature was 

 strongly attracted as above mentioned. 



The largest amount of magnetization was bestowed upon the small 



