324 TENTH ANNUAL REPORT OF 



of a galvanic arrangement can be determined witli accuracy. Un- 

 fortunately, such methods hitherto have been but little known, and 

 thus it is that we have descriptions of the useful effect of many differ- 

 ent combinations of galvanic aiojmratus, but none such as give an 

 accurate comparison of different apparatus, and a consequence is that 

 we are frequently deceived in their value. 



For determining the constants of a battery, it is essential to under- 

 stand, in the first place, with reference to the unit of current, whether 

 the observations made for that purpose are comparable with other 

 observations at different places with different instruments. To render 

 such a unit popular, it should be accessible to practical men, who though 

 acquainted with the principles of electricity, are unable to enter into 

 the specialities of the science ; hence it is fit to select such a unit only 

 whose definition is easily and generally comprehensible ; moreover, 

 the unit should be such, that the determination of the force of the 

 current for obtaining it may be accomplished with the least possible 

 apparatus. 



Considered in this light, the unit first brought into use by Jacobi 

 has by far the preference. I will endeavor to justify this opinion. 



§ 6. Beduction of PouilleVs unit to chemical measure. — To com- 

 pare the indications of any compass with Pouillet's unit^ we must 

 have a thermo-electrical element exactly equal to that used by him ; 

 and for that purpose it is necessary that the entire resistance of the 

 circuit, including the wire of the compass or multiplier, should be 

 equal to the resistance of a copper wire 20 metres long and 1 millimetre 

 thick. But the current which such a thermo-electrical element pro- 

 duces under the indicated conditions is exceedingly feeble, or at least 

 much weaker than the current of hydro-electric batteries, which yield a 

 practical useful effect; and in instruments with which ordinarily the 

 force of the current of hydro-electric batteries is measured, as tangent 

 compasses, sine compasses or Mohr's torsion galvanometer, Pouillet's 

 unit will produce but a very small deflection. This unit produces, for 

 example, in Weber's tangent compass, having a ring 40 centimetres 

 in diameter, a deflection of from 5 to 7 minutes; in Mohr's torsion 

 galvanometer, a deflection of about 1^ degree; thus it is requisite to 

 have very small subdivisions of a degree in these instruments with 

 accuracy, for determining this angle of deflection with sufficient ex- 

 actness to make the angle itself, or its tangent, the unit in measuring 

 strong currents. 



Since the instruments do not admit of sufficiently accurate reading 

 of such small angles, an indirect method must be introduced. The 

 following, perhaps, is the simplest for this purpose : 



Pass the current of the thermo-electrical element, serving as unit, 

 through a multiplier, and observe the deflection produced : suppose 

 it is 16°, the entire resistance here is equal to the resistance of a 

 copper wire 20 metres long and 1 millimetre in diameter. 



Now pass the current of a hydro-electric element through the same 

 multiplier, but insert, in the form of platinum or German-silver 

 wire, resistance until the deflection is as great as that produced by the 

 thermo-electrical element, or until it amounts, as before, to 16°. 



