6 Magnetometer for showing the Influence of Temperature. 



For demonstrating the influence of temperature on mag- 

 netic substances, I have devised a magnetometer which is 

 shown in the accompanying figure {Plate I.) two-thirds the 

 actual size. 



The instrument consists of a declination needle freely- 

 suspended from a double fibre of untwisted silk over a disk 

 of brass. One end of the needle is thickly covered with 

 spun silk to prevent the weakening of its magnetism by 

 close proximity to the heated substance under examination- 

 The excursions of the needle are limited in both directions 

 by pins inserted a little distance apart on a diameter near 

 the edge of the disk. The disk is pivotted on the end of 

 an arm, to which it can be clamped firmly by means of a 

 milled screw when the needle is drawn out of the magnetic 

 meridian. Three binding screws are mounted at equal 

 distances from each other round the circumference of a 

 circular table, which has an independent movement round 

 the disk. The magnetic substances are held in loops of 

 platinum wire fixed by the binding screws to the table, and 

 the properties of the specimens can be examined in 

 succession. 



The action of the instrument is as follows : — The needle 

 is drawn out of the magnetic meridian from 15 to 20 by 

 turning the disk on its axis. The magnetic substance is 

 then brought round towards the needle until equilibrium is 

 established between its magnetism and the horizontal com- 

 ponent of the earth's magnetism. The magnetic substance 

 is heated by a small gas flame from below ; when the needle 

 recedes from heated iron, magnetite, and nickel, and advances 

 again when the source of heat is removed ; thereby indi- 

 cating a decrease of magnetic power for these substances. 

 On the other hand, the needle advances towards cobalt 

 when heated, and recedes when the metal is cooled, by 

 reason of its enormous surface resistance, which only 

 disappears, as I have said, under . the . action of powerful 

 magnetising forces. 



