322 Mr. T. Gray on the Experimental Determination 



a globe of the same magnetic moment as the earth. He 

 found that this bar must contain 3'55 kilogrammes of the 

 steel of which his magnets were made. 



No experiments, however, have been made and published 

 hitherto (so far as I know) having for their object the deter- 

 mination of the magnetic moments of steel magnets of different 

 tempers and tempered by different methods, or which give 

 information as to the permanence or non-permanence of the 

 magnetism of such bars when left undisturbed for any consi- 

 derable time. The experiments described below were under- 

 taken with the view of supplying some approximately accurate 

 information on these points, and also as to whether a hard or 

 soft quality of steel gave the stronger magnets. They were 

 performed in the Physical Laboratory of the University of 

 Glasgow. The experiments on the effect of temper were all 

 made on small bars cut from a wire of soft, carbon steel. 



The apparatus used is shown in the accompanying diagram 

 (PLY.). M represents the magnetometer, which is a reflect- 

 ing instrument consisting of a small mirror about one centi- 

 metre in diameter, carrying, cemented to its back, four small 

 needles about 0*8 centimetre long, and suspended by a single 

 silk fibre ten centimetres long, which passes down a narrow slit 

 cut in the front of a wooden upright fixed to the base. This 

 slit terminates in a small cell, in which the mirror hangs. The 

 slit and cell being closed in front by a glass plate, a dead-beat 

 arrangement is obtained similar to that of Thomson's reflecting 

 galvanometer. B B is a bar of wood capable of turning round 

 the vertical axis E, which, by means of a brass spring S is 

 made to bear against two brass V's, one of which is fixed 

 to the upper and the other to the under side of B B. 

 A A are two arms of wood (shown in plan at the foot of the 

 diagram), each of them fixed to BB by means of two thin 

 wooden strips W. As will be seen from the plan of the arms, 

 these strips were, in every position in which they were placed, 

 in a vertical plane passing through the axis R. 



The upper side of the bars A A was on a level with the 

 centre of the mirror ; and along the centre of them a small 

 V-groove was cut, the line of which was arranged to pass 

 through the centre of the mirror. The axis of a magnet 

 placed in this groove could evidently, by turning the arms 

 A A, be caused to make any desired angle with the magnetic 

 meridian ; and hence the instrument could be used either as a 

 sine or tangent instrument. L is an ordinary galvanometer- 

 lamp, and M a scale of half-millimetres placed at a distance of 

 one metre from the plane of the mirror. 



The image of a fine wire, fixed vertically at F, was brought 



