28 Mr Hearle, Notes on a Vibration Magnetometer 



meters free from the defects just mentioned, and, after several 

 attempts, I designed the instruments described in this paper. The 

 magnetometers have been in use in my class at the Cavendish 

 Laboratory, Cambridge, since 1899, and have proved very efficient 

 in the hands of my students. A description of them may there- 

 fore, perhaps, be welcome to other teachers. 



The vibrating system is shewn drawn to scale in the figure. 



The magnet is a cylinder, 1*50 cm. in length and "14 cm. in diameter, 

 of the " silver steel " used for making small tools, the steel being 

 rendered " glass hard " by quenching at a bright red heat. The 

 magnet is magnetised between the pole-pieces of a small electro- 

 magnet and its magnetic moment is about 3 C.G.s. units. In order 

 to make a great increase in the time of vibration the magnet is 

 attached to a brass plumb bob. The main part of the bob is 

 1'6 cm. in length and "8 cm. in diameter, the total mass of the 

 system being about 8"5 grms. The lower part of the bob is turned 

 down to a smaller diameter, and into this part are fitted the 

 magnet and a pointer of aluminium wire 5 cm. in length ; the 

 lower end terminates in a sharp point. The upper end of the bob 

 is fitted with a small eye, made by soldering a loop of fine wire 

 into a small hole drilled along the axis of the bob ; to this eye the 

 suspending fibre is attached. 



If the system be carefully constructed and suspended, the 

 centre of the magnet will lie very accurately in the vertical line 

 through the point of the bob. This circumstance renders the 

 magnetometer very suitable for experiments in which the distances 

 are marked off on a sheet of paper fastened to a drawing-board, 

 since the centre of the magnet can be placed very exactly over any 

 given point. (See § 8 below.) 



The bob is some 50 times as heavy as the magnet itself, and 

 thus the system is not perceptibly drawn aside in a non-uniform 

 field, unless the space-rate of variation of the field is very great. 

 The system therefore vibrates quite steadily about a vertical axis 

 without developing any pendulous motion. 



