14 Mr. Donovan on Galvanometric Deflections 



will be the eastern deflection, and the more slowly will the needle come round 

 to the west ; because, in that case, the bismuth points, abundantly suppUed 

 from the mass, do not lose, and the antimony does not gain heat rapidly. 



When the rubbing surface of the antimony is very small, as a fine point, 

 there may be no eastern deflection, because the point immediately assumes 

 nearly the same temperature as the hot mass of bismuth, before the inertia of 

 the needle can be overcome by the tendency to eastern deflection. If the sur- 

 face of the antimony be relatively very large, and the mass considerable, the 

 east deflection is trifling, and the inversion rapid ; for the small surface of bis- 

 muth is almost immediately cooled nearly to the temperature of the antimony. 

 If the bismuth, at a low heat, be rubbed against the cold antimony, there will 

 be no eastern deflection ; and therefore there can be no reversal: the deflection 

 natural to the metals prevails, because the difference of temperature is not ade- 

 quate to overcome that tendency : and hence the needle at once moves to the 

 west. 



Thus in such cases, and there are many, attrition, instead of reversing the 

 deflection caused by thermo-contact, induces and continues a deflection in the 

 same direction, provided the temperature of the heated metal be adequate, and 

 maintained. Attrition has nothing to do with the reversal, until, by gradually 

 equalizing the temperature of the metals, it permits another Law (xiv.) to 

 come into operation, which, it so happens, has a contrary effect to that of unequal 

 temperature. In fact, attrition, in this case, instead oi causing a reversal of deflec- 

 tion, induced by thermo-contact, merely permits by its continuance a restoration 

 of a deflective tendency natural to these metals when rubbed against each other, 

 their temperatures being at the time equal, or not very different. Even when 

 the temperatures are very different, they may not be adequately so: there are 

 instances wherein the natural deflective tendency is so strong that it will pre- 

 vail. Thus when bismuth and arsenic sufi'er attrition against each other, unless 

 the former be 212°, and the latter below 50', the eastern deflection will not be 

 produced. 



Generally, the deflective energy developed by attrition of two metals, at 

 very unequal temperatures, transcends and therefore overpowers the opposite 

 deflecting energy which is produced by attrition of the metals when there is 

 very little or no difference between their temperatures. But in proportion as 



