Thermo-Multiplier. 547 



not, generally speaking, proportional to the forces producing 

 them, and that for the most part angular spaces at greater di- 

 stances from zero correspond to increments of force greater 

 than for equal spaces near zero. Thus to cause the needle 

 to advance from 25° to 30° requires a force greater than to 

 make it deviate from 0° to 5°. Also the force indicated by a 

 deviation of 30° is more than six times the force indicated by 

 a deviation of 5°. M. Melloni has pointed out an ingenious 

 method of comparing the values of the different parts of the 

 scale. This consists in employing two constant sources of 

 heat to affect the opposite extremities of the pile, and after 

 observing their separate effects, noting their joint effect, which 

 will not generally be equal to the arithmetical difference of 

 the others. Thus let one source of heat force the needle in 

 a positive direction to 30° on the scale, and a second source 

 of heat acting separately produce a negative deviation of 25°, 

 the effect of both acting at once will not be a positive deviation 

 of 5° merely, but probably will indicate some greater number, 

 as 6° or 7°. Thus, a true scale of degrees equal in value to 

 those near zero may be constructed. 



Another mode of estimating the indications of the instru- 

 ment has been used by M. Melloni, and it is one particularly 

 adapted to our researches. It likewise gives much more uni- 

 form results than might have been anticipated. Instead of 

 noting the ^wfl/ or stationary deviation due to any heating 

 cause, it is sufficient if vi^e note the arc through which the 

 needle \s Jirst impelled, and employ a table of reduction, in- 

 dicating the relation subsisting between the dynamical effect 

 or first arc of impulsion, and the statical effect or that of final 

 deviation*. My experiments have given the following results, 

 and the last column indicates the actual intensities corre- 

 sponding to the statical deviation in second column. 



Dynamical effect, or first Statical effect, or 



arc passed over. Permanent deviation. Intensity. 



18.0 19.45 21.75 



20.0 21.5 24.3 



£• See Scientific Memoirs, vol. i. p. 329. — Edit,] 

 2 Y 2 



