1879.] On Dynamo- and Magneto- Electric Machines. 



295 



When the straight cores were placed on the poles'of the \J magnet, 

 and a current passed round the latter, attraction ceased the moment 

 the battery contact was broken ; but if, while the current was passing, 

 the armature was placed on the poles of the cores, the whole system 

 was firmly held together, though the current no longer flowed. 



There would appear to be no limit to the length of time during 

 which the stored-up magnetic force exerts itself in such metallic 

 circuits (closed, or nearly so) as are described above, for it was 

 found that, after periods varying from one to fourteen days from the 

 time of a momentary passage of an electric current round the cores, 

 the attractive force was as great, or even greater, than at the first 

 moment. 



A small electro-magnet, [J -shaped, with limbs 6 inches long, having 

 a core of f-inch iron, and helices consisting of 4 layers of No. 16 

 covered copper wire, had for its armature a similar |J core uncoiled. 

 The uncoiled \J was hung up, and the electro-magnet held beneath it, 

 the poles of each being opposed : a current from four Bunsen cells 

 was then sent through the coils for a few seconds. Not only did the 

 electro-magnet (weighing, with its coils, several pounds) remain 

 firmly attached to its armature, but the hanging on to it subsequently 

 of 8 pounds additional weight failed to detach it. 



A further proof of the large amount of magnetism held captive in a 

 circuit thus closed was afforded by the following experiments. On 

 connecting the ends of the wires from the helices with a galvanometer 

 and resistance-coil, deflections varying from 40° to 90° were obtained 

 with a resistance of 1,700 ohms in circuit each time the armature was 

 forced away from the poles of the large (J magnet, after the passage 

 of a current from four cells of a few seconds' duration. By careful 

 manipulation, sparks between the ends of the helix wires were also 

 obtainable each time the closed magnetic circuit was opened. (In one 

 case a week had elapsed betwixt the passage of the current and 

 obtaining of the spark.) 



In all these experiments, when the circuit was completely closed 

 there was no external magnetism apparent, but on slightly breaking 

 contact between the poles and armature, magnetic poles could be 

 detected. Slight irregularities of the surfaces in contact likewise 

 caused the development of poles. 



A heavy magnetic needle, 4 feet distant from the magnet, if 

 deflected by the \J magnet, uncovered by its armature, 45°, would fall 

 back to 5° when the current ceased. If the poles were now covered 

 by the armature, the needle went to 0°. Passage of the current from 

 four cells would now give a deflection of about 38° ; on the current 

 ceasing the needle would come back to 0°, and rise again to 5° on 

 removal of the armature : but if, instead of immediately pulling off the 

 armature, the two ends of the wires of the helices were connected 



