thrrmo-(;eotr()1'Ism 



511 



a liiuii ; for the twisted or^an resists further distortion 

 and a balance is struck when the twisting and untwist- 

 ing forces are equal and opposite. In this state of 

 equilibrium the effect of an external agent, say of varia- 

 tion of temperature, will l)ring about an upset of the 

 balance. The torsion will be increased if the external agent 

 induces au enhancement of geotropic action ; it will, 

 on the other iiand, be decreased when it induces a 

 din)inished r*^action. 



A physical analogy will make this point clear ; imagine 



a small magnetic needle 

 suspended by a thin 

 wire ; the earth's directive 

 force is supposed to be 

 annulled by the well 

 known device of a com- 

 pensating magnet. ' A 

 second and larger magnet 

 M is now placed at right 

 angles to the suspended 

 needle ; N will repel 71 

 and attract s, and a 

 deflection will be pro- 

 duced, the deflecting 

 force of the magnet M 

 being balanced by the 

 force of torsion of sus- 



Fl«. 184. — Magnet M causeK deflection of the pending Wire (Fif. 1 S4 ). 

 needle n .«, suspended V)y a thin wire. Increase 

 of magnetisation of M increases deflection, while 

 decrease of magnetisation diminishes the 

 deflection. 



The state of equilibrium will however be disturbed by 

 variation of the magnetic force of M. It is known that 

 a rise of temperature diminishes >nagnetisation while 

 lowering of teini)erature increases it. Hence the deflecting 



