1879.] 



On Dynamo- and Magneto-Electric Machines. 



293 



fitted approximately close to the poles of the U magnet, whose faces it 

 completely covered when placed upon them. 



Other armatures and magnets were employed, the form of which 

 we propose to describe in a future paper. 



The iron of which the [J and the straight cores were made was 

 found to be exceedingly plastic as regards molecular magnetic polarity. 

 In a few seconds after the cessation of an electric current from twenty 

 quart Bunsen cells acting through the above helices, they were in- 

 capable of attracting and holding even fine iron filings. 



The |J magnet tested with a suspended magnetic needle was found 

 to retain some magnetic polarity after many days ; in fact, it is 

 doubtful if the magnetism ever entirely disappeared, except when the 

 core was subjected to special treatment. 



On the other hand, the straight cores lost their induced magnetism 

 more rapidly, and when, having been demagnetized either by time or 

 by the mode described further on, they were placed in the line of the 

 magnetic dip, they showed poles in accordance therewith ; and on re- 

 versing the position of the core, these poles were immediately reversed 

 without its being necessary to resort to striking the bars or other 

 means of putting them in a state of vibration. It was thus demon- 

 strated to our minds that if iron of similar quality, and in this form, 

 were made use of for the electro-magnet cores in a dynamo-electric 

 machine, the initial force producing the electric currents of the machine 

 could not be due to residual magnetism, but rather to the lines of 

 magnetic force of the earth. 



The current from four Bunsen cells sent round the U magnet fixed 

 the armature so firmly that it could not be pulled, or even slid off, by 

 the utmost exertion of one man's strength. 



On breaking battery contact, if both poles were completely covered, 

 a direct pull failed to separate armature and magnet. The armature 

 could, however, though with difiiculty, be slid off ; the difficulty of 

 movement greatly increasing as the edge of the poles was approached. 

 For instance, on attempting to slide the armature off the north, the 

 south, or both poles, the resistance became greater as the point of final 

 communication between the poles through the moving armature was 

 approached. This was found to be the case whatever time had passed 

 between the rupture of contact and the first movement of the arma- 

 ture. (Sometimes many days elapsed.) In very many experiments it 

 was found, moreover, that, provided neither pole had been completely 

 uncovered, on sliding back the armature to its normal position, the 

 magnet, which with its stand and coils weighed over 58 lbs., could be 

 lifted by it. 



A current from four Bunsen cells, almost momentary in duration, 

 sufficiently magnetised the core to produce all the above effects. 



If, whilst the current flowed round the U magnet, the armature 



