250 Mr Peake, A Novel Instrument for Illustrating 



A Novel Instrument for Illustrating the Magnetic Properties of 

 Iron. By A. H. Peake, M.A., St John's College. 



[Bead 29 January 1906.] 



The instrument here described is one which was designed by 

 the author in order to illustrate, with as simple apparatus as 

 possible, the phenomenon of magnetic hysteresis. 



This instrument, of which a view is given in Fig. 1, is self- 

 contained, that is to say, no other apparatus or instruments are 

 required in conjunction with it, neither does it require any electric 

 current, the magnetising forces being produced by permanent 

 magnets which form part of the instrument. 



Principle of the Instrument. 



The principle on which it is based is as follows : — 

 If a sample of iron, which is very thin in proportion to its 

 length, is suspended in a uniform magnetic field, the magnetising 

 force, to which the sample is subjected, is equal to the resolved 

 component of the field in the direction of the length of the 

 specimen, and the induction produced is proportional to the turn- 

 ing moment experienced by the sample, when the plane of the 

 sample makes but a slight inclination with the plane normal to 

 the lines of force. In this particular instrument the turning 

 moment is balanced by a gravitational control, and the same 

 pointer serves to read on two different scales both the angle of 

 inclination of the specimen to the field, and hence the magnetising 

 force H, and also the angle turned through from the direction 

 assumed when under the action of gravity alone, this latter angle 

 being a measure of the induction B produced. See Figs. 2 and 3. 



Details of the Instrument. 



The sample used is a strip of sheet iron, or wires arranged side 

 by side so as to approximate in form to a strip ; they are three 

 inches in length, and may be of any width up to five-eighths of an 

 inch. They are supported in a cradle, to which a long aluminium 

 pointer and an adjustable brass control weight are attached ; the 

 cradle with the pointer and control weight are carried on two steel 

 points, which are pivoted, the one in an agate cup of conical 

 shape, and the other in a V-shaped trough of agate, so that after 

 removing the cradle for the insertion of a sample, the steel points 

 are easily replaced in their bearings in exactly the same position 



