326 TENTH ANNUAL REPORT OF 



known also how many of Pouillet's unit correspond to the observed de- 

 flection of the rheometer. Pouillet's unit has been used here only 

 nominally ; the deflection of the rheometer alone has, in fact, been 

 compared with the chemical effect, and there is no reason why this 

 comparison should not be adhered to. 



§7. Bedudion of Weber' s unit to the chemical measure. — The defi- 

 nition of Weber's absolute measure of the force of a current is by no 

 means so simple as to encourage the hope of making this unit easily 

 very generally comprehended. This inconvenience, however, might 

 be disregarded^ if the determination of the force of the current were 

 easily derived from this absolute measure. 



If a Weber's tangent compass (which should not be less than 40 

 centimetres in diameter) be used in getting the angle of deflection 

 which a current produces, it is made to appear stronger in absolute 

 measure, as expressed by the formula, 



T r tang, w 



According to this formula the value of the force of the current is very 

 easily obtained, if the correct value of T be ascertained ; that is, if 

 at the place of observation the horizontal part of the intensity of the 

 earth's magnetism, expressed in absolute measure, be known. 



The determination of T (Miiller's Lehrbuch der Physik, 3d Aufl. 

 2 Bd.) has for special physicists no great difficulty, but for many ar- 

 tisans who wish to determine the power of their batteries it is too 

 complicated ; at least it is more difficult than the comparison of the 

 data of a rheometer as made by Jacobi, with the chemical effect of the 

 current. It would not be necessary to determine the value of T by 

 experiment at the place of observation ; it might be derived from the 

 magnetic chart of Gauss, if it were certain that at the place of (Ob- 

 servation the effect of the horizontal magnetism of the earth was not 

 modified by iron deposited in that locality, which would produce a 

 considerable deviation from T. For instance, we have from Gauss' 

 chart, as well as from direct observation made in the open air, that 

 for Marburg T=: 1.88, while Kasselman found the value of T, in the 

 locality in which he instituted the experiments for comparing the 

 force of the currents of different galvanic batteries, equal to 1.83, 

 (tJber die galvanische Kohlenzink Kette von Kasselman : Marburg, 

 1844, p. 75) ; hence it is unavoidably necessary to determine the 

 value of T in the locality in which the experiments on the strength 

 of currents are instituted. 



Weber's unit decomposes 0.000009376 grammes of water in one 

 second ; in one minute 0.0005G256 grammes ; or_, what is the same, it 

 yields 1.0477 cubic centimetres of detonating gas per minute. 



To determine the force of a current according to this measure, a 

 tangent compass of Weber is needed^, whose ring should not be less 

 tlian 40 centimetres in diameter, while rheometers of different kinds 

 can be used if the unit of the current yielding one cubic centimetre per 

 minute of detonating gas be adopted. 



