THE SMITHSONIAN INSTITUTION. 325 



The whole resistance which the hydro-electrical current has now to 

 overcome must he determined and reduced to that of copper wiie. 

 Suppose it is equal to the resistance of a wire 1 millimetre in diame- 

 ter and 22,000 metres long. 



By making the entire resistance less by the removal of wire, the 

 current will become stronger in equal measure. Make the resistance, 

 for instance, 200 times less, so that the entire resistance to be over- 

 come by the current of the hydro-electrical element is equal to the 

 resistance of a normal copper wire only 110 metres long ; the current will 

 now be 200 times stronger than that which produced a deflection of 

 16° in the multiplier. This current will produce a considerable de- 

 flection in each instrument adapted to measuring stronger currents, 

 as a Weber tangent compass ; let it be 19°. 



Thus a current which indicates in the tangent compass an angle of 

 deflection of 19°, of which the tangent is =z 0.344, is 200 times as 

 strong as the unit of the current, thus we have for the tangent of the 

 angle to which the unit corresponds — 



5:?^ = 0.00172. 

 200 



By this result all the indications of the tangent compass can be 

 easily reduced to Pouillet's unit. 



Pouillet used, not a tangent compass, but a eompass of sines, in all 

 his researches on this subject. 



To decompose one gramme of water in one minute, the current 

 passed through the water must have a force = 13,787 of Pouillet's 

 unit. Each gramme of water yields 18G2.4 cubic centimetres of de- 

 tonating gas (at 0° and a pressure of 7G0 metres) ; hence to obtain 

 one cubic centimetre of detonating gas per minute, a force of current of 

 i-lll^ r= 7.4 Pouillet's unit is necessary. 



The above examples will sufiice to show that the reduction of the 

 data of a rheometer for stronger currents to Pouillet's unit can be 

 obtained only by a whole series of operations by no means simple. 

 First, the resistance of the thermo-electric element, and of the mul- 

 tiplier, must be determined, and so much resistance must be added 

 that the sum of the resistances shall have the value given above ; then 

 the resistance of the conductor of the hydro-electrical element must 

 be found, and after inserting as much resistance in its circuit, the 

 quantity of this resistance is to be determined ; then the entire re- 

 sistance must be reduced to an aliquot part, and the corresponding 

 deflection of a rheometer used for stronger currents observed, &c. The 

 end here is attained only through a circuitous process, and errors of 

 observation are unavoidable in each operation, which affect the final re- 

 sult ; the complexity of the process also has a prejudicial influence on 

 the accuracy of the determination. 



The above comparison of Pouillet's unit with the chemical effect pro- 

 duced^ gives us the means of easily converting the data of a rheome- 

 ter into this unit ; we have only to pass the current simultaneously 

 through the rheometer and an apparatus for decomposing water, to 

 determine how much detonating gas will be evolved while the rhe- 

 ometer indicates a certain number of degrees. Since each cubic centi- 

 metre of detonating gas corresponds to 7.4 of Pouillet's unit, it is 



