Intelligence and Miscellaneous Articles. 397 



heating the mercury, the end of the column in the stem moves. The 

 delicacy of the thermometer varies with the height of the mercury, 

 but according to a simple law which is deduced from calculation or 

 from observation. 



A very fine platinum wire is stretched in the reservoir from the 

 top to the cistern ; its ends are fused into the glass and connected 

 with the poles of a battery; it transmits the current through the 

 mercury without resistance ; offering through the water a resistance 

 x, it developes an amount of heat which only affects the water of the 

 thermometer, which does not heat the metal, and which cannot be 

 transmitted downwards owing to the feeble conductivity of the liquid. 



(1) By raising or lowering the mercury, the length and resistance 

 x of the platinum wire are diminished or increased at pleasure. The 

 apparatus thus becomes a rheostat analogous to that of Pouillet, but 

 more convenient and better, inasmuch as the wire, being immersed in 

 water, is but little heated, and serves for currents whether strong or 

 weak. 



(2) The heat developed by the passage of a current may be mea- 

 sured. It equals p(t' ' — t), the product of the weight of water into 

 the increase of temperature ; the weight is sxd. The increase of 

 temperature is measured by the variation of the volumes, or the num- 

 ber n of divisions through which the thermometer-column has moved, 

 divided by the volume sx, and by the coefficient of expansion k. Then 



,., ,n sxd d 



which signifies that the heat given up, apart from the corrections, 

 which I here neglect, may be measured by the number of divisions by 

 which the thermometer-column has moved ; n is independent of the 

 height of the mercury. A simpler result could not be arrived at. The 

 measurement of n is made with the ordinary precautions. 



(3) This heat n is proportional to the resistance x, which is deter- 

 mined by the square of the intensity. We have then 



>=\/l- 



The instrument may thus be used as a galvanometer ; the smaller the 

 ratio of the sections of the stem and of the reservoir, the more delicate 

 it is. In fact it is a thermo-rheometer. 



(4) Replacing i by its value, we get 



A 2 x 



Making the determinations of n with different values of x, A and R 

 may be calculated ; that is, the electromotive force and the resistance 

 of a battery may be determined without the aid of any other instru- 

 ment than the thermo-rheometer. 



(5) What has been said applies to induction-currents as well as 

 to the current of batteries. The former have hitherto not been much 

 investigated : as they are alternately contrary, it is impossible to se- 

 parate them accurately ; and in general their effects neutralize them- 

 selves. The calorific effect alone is independent of the direction of 

 the currents and unaffected by their interruptions ; and, finally, the 



