1880.] 



On Magnetic Circuits in Electric Machines. 



291 



tact with the bichromate cells, the closed magnetic circuit was found 

 capable of sustaining the 4 lb. weight. 



By successive additions of 2 oz. weights, made at intervals of a few 

 minutes, the weight hanging to the armature was raised to 5 lbs., after 

 which the attempted addition of 2 oz. caused the disruption of the 

 system. 



The experiment was repeated under similar conditions, but with 

 slightly extended intervals of time between the additions o^ the 2 oz. 

 weights. The magnet in closed circuit was made to hold 4 lbs., 

 4J lbs., 4^ lbs., 4 lbs. 14 oz., 5 lbs. 2 oz., the time taken in all, for 

 the successive additions, being ten minutes. The system was then 

 left for twelve hours, when by additions of 4 oz. at intervals of a 

 few minutes the weight sustained was increased to 6 lbs. 4 oz. Eleven 

 hours later, this was further increased to 7 lbs. 6 oz., and two hours 

 afterwards to 8 lbs. 2 oz. 



A still smaller electro-magnet, weighing with its coils 5 oz., and 

 having an armature consisting of a very thin slip of soft iron, when 

 excited by one of the bichromate cells, could not be made when in closed 

 circuit to sustain 1 J lbs. at the moment of breaking the voltaic circuit. 

 It, however, sustained 1 lb. with ease. The latter weight was there- 

 fore suspended, and the cell wires removed after the closed magnetic 

 circuit was completed. By successive additions of 2 oz. weights at 

 short intervals of time (five minutes to ten minutes each), this small 

 magnet could be made to sustain 2 lbs. 2 oz., but the addition of 1 oz. 

 beyond this weight at once separated the armature and magnet. It 

 was thought that a longer interval of time should, as in the former 

 instances, enable the magnet to sustain a still greater weight. It was 

 therefore brought into closed circuit, as before, and made to sustain 

 2 lbs. 2 oz. in the manner just related, and was thus left for twelve 

 hours. Successive additions of 2 oz. were then made to the hanging 

 weight until it reached 2 lbs. 14 oz. Twenty-four hours afterwards, 

 4 oz. more were added, bringing the entire weight suspended to 50 oz. 



This small, soft iron magnet which, at the instant the voltaic current 

 was withdrawn, was totally unable to sustain five times its own weight, 

 was thus, by gradual growth of its magnetic force, enabled to hold ten 

 times its own weight. 



In the course of these experiments it was remarked that the longer 

 the period the soft iron remained in closed magnetic circuit the more 

 magnetically ductile did its molecules appear to become. An electro- 

 7nagnet, which had been for a few days in closed circuit, could after 

 rupture of the circuit be made to sustain weights in a fresh closed 

 circuit at much shorter intervals of time than if it was magnetized, 

 after being for some time with its poles uncovered. The direction of 

 the battery current with reference to the residual magnetism of the 

 electro-magnets appeared to be of no moment. A magnet which had 



