IQ Mr. Donovan on Galvanometric Deflections 



perature, or to an equality of temperature, were to immerse them in water, 

 whether hot or cold. The necessary cold was sometimes attained by keeping 

 masses of ice in the water. To produce equality of temperature in this man- 

 ner is not so easy as might be imagined — so small is the difference that will 

 give evidence of its existence. If ice be used, different parts of the water will 

 be at temperatures not exactly the same. Even when the water is hot, the 

 vessel, or tlie thick parts of it, will vary ; and by contact the metallic hemis- 

 phere may participate in the difference, or the wooden handles may defend 

 parts of the metal. The best way is to allow the hemispheres and handles to 

 remain an adequate time in the water, using almost constant agitation, until 

 centrally and externally they be of the same temperature as the water. 



I have sometimes used the expression " adequately hot," or " adequately 

 cold," to signify that decided effects can only be produced by a considerable 

 difference of temperature. In a few instances I found that the hemispheres acted 

 better when their flat faces were wet ; and this does not alter their indications. 

 Instances occur in which the deflective energy of the metals is so feeble that 

 attrition, effected by the hemispheres held in the hands, is inadequate ; in such 

 cases I had recourse to a piece of revolving machinery, which will be described 

 hereafter, and thus the metals were made to rub against each other with the 

 greatest rapidity. I now proceed to explain and amplify these different Laws. 

 Laws I., II., III. On the first, second and third Laws, there is little to re- 

 mark. It is not metals alone that exhibit the phenomena ; many of their ores 

 exhibit the same powers, but generally with less energy. Newly burnt box- 

 wood charcoal, rubbed against a revolving plate of bismuth, acted on the gal- 

 vanometer so energetically, that the needle traversed the whole circle with 

 vivacity. That peculiar carbon, also, which is deposited on the interior of iron 

 gas retorts long in use, acts as a metal. Either of these forms of carbon com- 

 ports itself with bismuth, in all respects, like antimony, with regard to the 

 direction of the needle, when acted upon by thermo-contact, or attrition. Gra- 

 phite also acts similarly. 



In appreciating the effect of temperature on an associated pair of different 

 metals, which produce deflection by contact, it has been supposed by Erman 

 that the temperature of the surrounding atmosphere is the standard at which 

 no deflection would be produced by contact, and that by raising or lowering 



