24 Mr. Donovan on Gakanometric Deflections 



become null ; the needle will stand at zero, for the polarity has been destroyed. 

 Or if the German silver rod had been originally heated throughout equally, as 

 in boilincr water, there will be no deflection when it is brought in contact with 

 the cold iron rod.* If the iron rod on the silver side be heated at one end, its 

 contact with the cold German silver rod will cause western deflection, and will 

 manifest polarity ; but, when it is heated equally throughout, it will be incapa- 

 ble of producing deflection. Thus these two metals agree in their incapability 

 xmder the circumstances stated. But if one end of either be hotter than its 

 other end, although both be hot, there will be corresponding deflection. 



But this incapability of producing deflection, which arises from equal diffu- 

 sion of high temperature throughout the whole rod, is not common to all me- 

 tals. With a bismuth rod on the zinc side, and an antimony rod on the silver 

 side, if one end of the bismuth be heated, it will, by contact or attrition with 

 the cold antimony rod, give Avestern deflection for the hot end, and eastern for 

 the cold end. If the whole of the bismuth rod be heated equally throughout, its 

 polarity will be destroyed; but the needle, instead of standing still at zero, will 

 pass to the east, because the rod no longer acts as such, but as a hemisphere, 

 on account of the great natural energy of these metals in thermotribo-electricity. 

 The case will not correspond when the rod of antimony is the heated one ; fur 

 whether it be heated at one end only, or throughout its whole extent, its con- 

 tact with the cold bismuth will be the same, viz. — eastern deflection on the zinc 

 side, and western on the silver side. 



If a bismuth rod, on the zinc side, be heated at one end, and made to act on 

 a cold iron rod on the silver side, there will be western deflection ; or eastern 

 if the heat be equal throughout. But if the iron rod on the silver side, be 

 heated at one end only, or equally throughout its whole extent, it will, in either 

 case, give western deflection by attrition or contact with the cold bismuth. 



If German silver be substituted for bismuth in the foregoing experiment, 

 and a tin rod for the iron, the same application of heat, as before, will produce 

 the same deflections ; the tin whether hot at one end only, or throughout, will 

 impel the needle in the same direction. Corresponding results will be obtained 

 with rods of tin and bismuth, or tin and antimony. 



* Attrition will sometimes give 2° or 3° west. 



