12 Mr. Donovan on Galvanometric Deflections. 



contact caused western deflection. I conceive that Professor Erman was 

 misled in these two cases by a circumstance which shall be hereafter noticed. 



It is obvious that, in my trials, the rheophores could not have caused any 

 interference ; for they were maintained at the same temperature as the hemi- 

 spheres or plates by being exposed to the same atmosphere or the same water, 

 and the incapability of the hemispheres to cause deflection proves equally that 

 of the rheophores when at the same degree of heat. 



Law V. The first part of this Law (marked 1) may be proved by many 

 examples. If a hemisphere of antimony and one of nickel, one cold, the other 

 hot, be brought into contact, the deflection will scarcely exceed 5^, but attrition 

 will send the needle perhaps 60°. The same observation may be made of 

 German silver associated with iron. Many other metals might be instanced. 

 The Law also holds good with regard to that peculiar kind of carbon which is 

 deposited in the interior of old gas retorts : if a large mass of this, which has 

 been long heated in boiling water, be brought in contact with a piece of cold 

 bismuth, a deflection of about 35° will be produced, but attrition will cause the 

 needle to start ofl" 20° or .30° farther. 



The second part of the Law (marked 2) is thus exemplified : when adequately 

 hot bismuth (as 200°) is on the zinc side, and antimony (below 50°) on the 

 silver side, if the metals, without being allowed to remain in contact a moment, 

 are at once made to rub against each other at the instant of their first applica- 

 tion, there will be a certain amount of eastern deflection ; and the needle, 

 having attained its maximum, will remain stationary. But let the attrition be 

 discontinued, and the contact maintained ; it will be found that the needle will 

 proceed still more eastward than before, perhaps to the extent of an additional 

 20° or 30°, proving that, in this case, contact was more powerful than attrition, 

 both being effected at the same temperatures. Eenewed attrition will then 

 suddenly reduce the deflection, because the two metals are now somewhat 

 nearer to equality of temperature ; contact will again increase it ; and this 

 alternation may sometimes be effected several times before the temperatures 

 are equalized. It thus appears that the effect of attrition is not to be viewed 

 as the mere result of simple contact. 



Laws VI., VII., VIII. These Laws include facts which appear to me to have 

 been misunderstood. As an example, let a hemisphere of bismuth, with its 



