314 Profs. J. A. Fleming and J. Dewar. 



On passing a current through the magnet coils a magnetic field 

 is created in the space just above the upper end of the inner 

 steel core, which is very divergent, and which is not only strong, 

 but varies very rapidly in an axial direction. 



This magnet was placed underneath a shelf on which stood a very 

 sensitive short-beam chemical balance capable of weighing to one- 

 tenth of a milligram. From one pan depended a fine wire passing 

 through a hole in the balance case and shelf, and served to suspend 

 a ball just within the field of the magnet. 



If the ball was counterpoised by weights and the magnet then 

 excited, the ball was subjected to an upward or downward force 

 which either decreased or increased its apparent weight, according 

 as it was diamagnetic or paramagnetic. If "W is the gain or loss in 

 weight on exciting the magnet, V the volume of the ball, H the 

 strength of the field at its centre, and dB./dx the rate of change of 

 the field in a vertical direction, then the equation 



981W = ^VH 



ax 



gives us the value of the apparent magnetic susceptibility (A^) in 

 air of the body weighed. 



In order to apply the method we require to know the value of the 

 field H at different parts along the vertical axis of the magnet and 

 also the value of the field for various exciting currents. 



A careful preliminary investigation on this point was therefore 

 made. Constant currents varying from 1 to 12 or 14 amperes were 

 passed through the magnet coils and by means of a small secondary 

 coil attached to a calibrated ballistic galvanometer the field strength 

 at various points in the vertical central axis of the magnet was 

 measured. These measurements extended from close contact with 

 the magnet core to a point about 10 cm. above the end surface of 

 the recessed polar end of the inner steel core. 



The distance in centimetres of any point from the end polar sur- 

 face of the inner steel core is denoted by x, this measurement 

 being made as exactly as possible along the central vertical axis of 

 the magnet. The strength of the field at this point in C.G.S. units 

 is denoted by H, and the space rate of change in the x direction by 

 dH/d*. 



The following tables give the exciting currents (A) in amperes 

 measured by a Weston ammeter, No. 3134, and the values of x and H. 



The value of dHjdx at any point can be at once determined from 

 the curve of H in terms of x. 



