DanielVs Cell as a Standard of Electromotive Force, 127 



tromotive force, and the present paper is a brief collection 

 of facts concerning the use of DanielFs cell for this purpose, 

 and its application in the measurement of currents of 

 various strengths. In all the experiments and methods cited 

 below the cell has been used only to create a difference of 

 potential, and not allowed to send any but a very small 

 current, the means employed for comparisons being a slight 

 modification of Clark's potentiometer, as shown in fig. 1 

 (Plate V.). On a stout mahogany board are fastened down 

 two boxwood scales, each 5 feet long, divided into inches and 

 tenths; these scales are fixed about J inch apart and parallel, 

 so as to form a groove between them. On these scales are 

 stretched two fine uniform German-silver wires A B, A! B', 

 about "013 inch in diameter, and having a resistance of about 

 1 ohm to the foot-run: one end of both wires is soldered to 

 a. thick copper junction-piece J, and the other ends respec- 

 tively to copper pieces connected to terminals M, M 7 . The 

 wire thus forms one length of about 10 feet stretched over 

 two scales. This forms the potentiometer wire ; its length 

 is divided by the scale into 1200 parts, and each tenth of 

 an inch can be divided by the eye into 10, making a pos- 

 sible division of 12,000. To the terminals M, M / are con- 

 nected five or six large-gravity Daniell cells, and the poles of 

 this battery are short-circuited by an Edison 16-candie lamp 

 of 240 ohms cold resistance. This enables the battery to keep 

 a more constant difference of potential between the terminals 

 M, M'. 



A very constant E.M.F. can be obtained by using small 

 accumulator-cells in series : the poles being joined by a 

 carbon-filament lamp, and the leads to the terminals M, W 

 taken off from the opposite sides of the lamp. 



It is easy to calculate what number of cells are required to 

 maintain a given difference of potential, say 2 volts, at the 

 extremities of the potentiometer- wire. For let n be the 

 number of cells, e the E.M.F. of each, and r the internal re- 

 sistance, and let II be the resistance of the lamp and poten- 

 tiometer-wire combined, and v the required difference of 

 potential desired at the terminals M, M'; then 



neR 



nr 



+ R' 



which determines n. If r is not known, it can be determined 

 by a second experiment, in which v is observed in the case of 

 a given number of cells. 



The current along this German-silver wire makes a fall in 

 potential at about the rate of 1 volt in 5 feet. On one side of 



