ON THE TIME-LAG IN THE MAGNETISATION OF IRON. 



299 



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C is the " compensating coil," which is wound on a small wooden 

 reel, of diameter 4.75 cm. and breadth about 9 mm. This could slide 

 along a groove in front of the magnetometer, perpendicular to the 

 magnetic meridian. Its action on the magnetometer is opposite to 

 that of the iron wire, so that it serves for compensating the so-called 

 lediate effect of the magnetisation on the magnetometer. 



T is the contact maker, consisting of a small dish half full of 

 cury and a metallic ball connected with a spring. As long as the 

 ball is kept ahove the mercury surface, there is no circuit, but when it 

 is allowed to descend, which it is always ready to do by means of the 

 spring, contact is at once made. 



K is the ordinary mercury switch for rapidly reversing the current. 



Gr is an ordinary tangent galvanometer for measuring the current 

 strength. 



L is the liquid rheostat which serves to vary the E. M. F. of the 

 main circuit gradually, so that the iron wire can be subjected to the 

 process of demagnetisation by reversals. The zinc plates have the 

 diameter of 5 cm. and are thoroughly amalgamated from time to time,, 

 when in use. The cylinder is nearly 45 cm. high. 



11 is a resistance box, and B is a battery of six gravity Daniel!'» 

 cells. One end of the wire from the battery is joined to the resistance 

 box and through this to one of the terminal plates of the liquid rheostat,, 

 while the other end is connected with the other terminal plate. The 

 main circuit includes the magnetising solenoid, S, the compensating coil, 

 C, and the galvanometer, G, and its ends are connected one with the 

 lowest plate and the other with the middle plate of the liquid rheostat, 

 the latter plate being moved up and down by means of a long string 

 passing over a pulley. Of course, the magnetometer, the galvanometer, 

 the liquid rheostat, the resistance box, and the battery are placed at 

 sufficient distances from one another to be beyond mutual influence.. 



