8 Mr. Donovan on Galvanometric Deflections 



that on which the deflection would have temporarily taken place, had the me- 

 tals been, throughout their mass, exposed to that temperature. If the portions 

 of the metals acted on be raised above the temperature of their remainders, 

 the deflection will be on the side of the magnetic meridian opposite to what 

 it would have been had these parts been reduced to a temperature below that 

 of their- remainders. 



Law XII. The deflection produced by thermo-contact or attrition will be 

 always reversed when the exciting metals connected with the extremities of the 

 galvanometer coil are transposed. 



Law XIII. When deflection is produced in consequence of the attrition or 

 contact of two metals, one of which is hotter than the other, the deflection will 

 in many cases change to the opposite side of the magnetic meridian if the 

 hotter metal be adequately cooled, and the cooler metal be adequately heated, 

 the contact or attrition being renewed. Any two metals which, when asso- 

 ciated, comport themselves in this manner may fail to do so when differently 

 associated. 



Law XIV. The deflection produced by the mutual attrition of any parti- 

 cular pair of metals will take place at all temperatures of these metals on the 

 same side of the magnetic meridian, provided that the temperature is equal or 

 nearly equal in both. As this direction of the needle is always the result of 

 the attrition of these particular metals, when they are in their ordinary state of 

 equality of temperature, it may be conveniently called the natural deflection of 

 any pair of metals. 



Law XV. The deflection caused by chemical action of a menstruum on 

 two associated metals has no observable dependence on, or connexion with, 

 that produced by thermo-contact or attrition of these metals. 



Law XVI. The agent developed by the attrition of two metals, even when 

 rapid, forcible, and long continued, does not manifest any decomposing influence 

 on chemical compounds. It is not conducted by aqueous liquids, even when 

 containing saline impregnations. 



To these sixteen laws, in the expression of which the language of hypothe- 

 sis has been avoided, I conceive may be referred the whole of the complicated 

 phenomena which thermo-contact and attrition of metals and other substances 

 produce ; at least as far as researches have yet disclosed. 



In the following statements, when reference is made to the galvanometer, 



