1 38 Professor Forbes on the Refraction 



of the galvanometer ; and, owing to the dynamical effect which 

 I have described of a first impulse, that it is improbable that it 

 amounted to half that quantity. 



13. Hence it becomes an object of interest to form some esti- 

 mate of the sensibility of the therm o-multiplier, compared to com- 

 mon thermometers. It would be difficult to give a precise mea- 

 sure of the degrees of temperature of the two extremities of the 

 pile,* but we may compare the effect of equal quantities of heat 

 upon this and another instrument. For this purpose I employed 

 two air thermometers of great delicacy ; one was the photometer 

 of Leslie, having one ball covered with lamp black, and exposed 

 to the same source of heat as the pile, whilst the other ball was 

 shaded. The other instrument was a vertical differential ther- 

 mometer, having a hemispherical reflector, intercepting a cone of 

 rays 2.50 square inches in section. I found it impossible to ope- 

 rate with small degrees of heat, which could not be reckoned ac- 

 curately on the air thermometers, owing to their tardy action ; 

 but, from several experiments, I concluded that the same quan- 

 tity of heat falling on the photometer ball and on the pile, moved 

 the liquid of the former through 1 °, and the needle of the multi- 

 plier through 4.°2. The degrees of the photometer being lOths 

 of 1° cent., one centigrade degree would correspond to 42° of the 

 galvanometer, (assumed of equal value throughout the scale.) 

 The experiment with the differential thermometer, being similar- 

 ly conducted, gave for the effects of equal quantities of heat, 1° 

 cent, to 62° of the multiplier. If we assume from these experi- 

 ments that a quantity of heat which raises an air thermometer 

 by one-fiftieth of a centigrade degree, affects the galvanometer 

 by 1°, since a quarter of a degree of the latter is a measurable 



* This might best be done by adapting a differential thermometer of extreme 

 delicacy, so that the balls might be in contact with the two extremities of the pile, 

 and the spaces round them filled up with copper filings, or some such material. 

 But the experiment could hardly be quite decisive. 



