20 Mr. Donovan on Galvanometric Deflections 



The first-named metal of each pair being on the zinc side, and the second 

 on the silver side, attrition at equal temperatures will cause western deflection ; 

 it will also be western whether the first or second named metal be hot, its asso- 

 ciate being cold. And when the rheophores are transposed at the binding 

 screws, the deflections will all be eastward, whether the associated metals are at 

 tlie same temperature, or one at a higher temperature than the other, uo mat- 

 ter which is hot or which is cold. It is therefore obvious that under no cir- 

 cumstances of these metals could attrition even simulate a reversal of the de- 

 flection caused by contact. 



It is on account of this class of metallic associations that Law xm. is not ex- 

 pressed as universal in its application. In the annunciation of that law it is said, 

 that " the deflection will in many cases change to the opposite side," &c., be- 

 cause the associations above described do not so change. Notwithstanding this 

 the law may be universal ; for it is easy to understand that whatever influence 

 the hot metal exerts on the cold one in order to produce a certain deflection, 

 the latter when hot may happen to produce the same effect with the former 

 metal when cold. 



From all that has been said, it is obvious that if the contact of two metals, at 

 unequal temperatures produce deflection in the same direction as that caused 

 by their attrition under the same circumstances, and the deflection is on the 

 side of the magnetic meridian, opposite to that which the attrition of the metals 

 would have occasioned if their temperatures had been equal, then a reversal 

 takes place which I have called simulated^ because it does not arise from any 

 opposition between the effects of contact and attrition, but from one effect su- 

 pervening on the cessation or diminution of another. Thus, the reversal is 

 passive, not active ; and no such phenomenon occurs as an active reversal. 



But when the deflections, produced by attrition and contact of the asso- 

 ciated metals at different temperatures, are all the same for the three conditions 

 which may take place at each extremity of the galvanometer coil, then even 

 simulated reversal is impossible. All this is very plain. 



Law IX. This law is founded on probability and experience, as well as on 

 the difficulty of admitting the truth of the common opinion. It is generally 

 believed that the mere condition of difference of temperature in the associated 

 metals is the cause of the deflections which their contact produces. It appears 



