1883.] ^51 rBarker. 



tive forces, provided the angle of rotation be small. A mirror attached 

 to the suspension of the needle enables these deflections to be accurately 

 read with a telescope and scale. A simpler instrument suffices when the 

 zero method is employed. In this case the two electromotors are simul- 

 taneously connected to the quadrants, their electrodes being reversed. If 

 equal, the deflection will be zero. If unequal, it will be equal to the dif- 

 ference. By varying the known electromotive force until the deflection 

 is zero, the two are again equal. 



While, in the direct method, the electromotive force is the quantity 

 which is measured, in the indirect method some other quantity or 

 quantities are measured, and the electromotive force deduced by calcu- 

 lation from the known relation between the quantities. "When, for example, 

 the current strength is measured on the galvanometer and the resistance of 

 the circuit is known, the law of Ohm enables the electromotive force to be 

 computed. In Wiedemann's method, the electromotor to be measured is 

 joined up with the standard battery, in circuit with a galvanometer, first 

 with the electrodes in the same direction, then reversed. The electro- 

 motive force required is then the product of the standard electromotive 

 force by the quotient of the difference of the current strengths divided by 

 the sum. Another method consists in putting the standard cell in cir- 

 cuit with a galvanometer, the resistances of both being known. The 

 standard cell is then replaced by the electromotor to be tested and the re- 

 sistance in circuit varied until the same deflection is obtained. The elec- 

 tromotive force of the standard cell multiplied by the ratio of the second 

 total resistance to the first gives the electromotive force required. The 

 electrometer methods have the advantage of not using the current of the 

 electromotor to be measured ; and hence any change in its condition due 

 to the current produced is avoided. 



From what has been said, it will be evident that the selection of the 

 standard cell is a matter of prime importance. The advantages of the 

 Daniell cell for this purpose are too well known to require elaborate state- 

 ment here. As used on closed telegraphic circuits and the like, two forms 

 have come into general favor. One of these is that employed originally 

 by Professor Daniell. It consists of a glass jar containing copper sulphate. 

 In which the copper plate is immersed, and of a porous cup containing 

 the zinc plate, a more or less dilute solution of zinc sulphate. The 

 other form is the modification first proposed by Varley and afterward 

 by Callaud, in which the porous cup is done away with, the differ- 

 ing densities of the two solutions being depended upon to keep them 

 separated. The copper sulphate solution is placed at the bottom of the jar 

 in contact with the copper plate. As the density of this solution when 

 saturated is 1.186 at 15° C. the solution of zinc sulphate ordinarily rests 

 upon it and in contact with the suspended zinc plate. But as the action 

 of the battery goes on and the zinc sulphate accumulates in the solution, 

 this later finally becomes heavier than the copper sulphate solution (the 

 density of a saturated solution of zinc sulphate being 1.44 at 15° C), and 



PROC. AMER. PHILOS. SOC. XX. 113. 4d. PRINTED MAT 5, 1883, 



