284 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



fore be determined by measuring these lengths, so that the ratio 

 of the two electromotive forces is then known. 



Standard cells. The Clark standard cell is described on page 

 1 6. Its electromotive force in volts at / C: is given by the 

 equation 



E = 1.4292 o.ooi23(/ 1 8) o.ooooo7(/ l8 ) 2 



The Weston cell is similar in every respect to the Clark cell 

 except that cadmium amalgam and cadmium sulphate are used 

 instead of zinc amalgam and zinc sulphate. The electromotive 

 force in volts of the cadmium cell (with concentrated solution) at 

 / C. is given by the equation 



= 1.0187 o.oooo35(/ 18) 0.00000065(7 i8) 2 



MEASUREMENT OF POWER. 



160. Measurement of power by means of the ammeter and 

 voltmeter. The power delivered to an electrical circuit may be 

 calculated from the equation P = El, when the current / in 

 the circuit and the electromotive force E between (across) the 

 terminals of the circuit have been measured. This method is 

 applicable only in the case of direct currents, that is, where the 

 current / and the electromotive force E are steady in value. 



161. Measurement by means of the wattmeter. The watt- 

 meter is a special form of electrodynamometer the connections of 



which are shown in Fig. 210. A 

 fixed coil of coarse wire B is con- 

 nected in series with the receiving 

 circuit to which the power to be 

 measured is delivered, and a sus- 

 pended or pivoted coil A of fine wire 

 is connected across the supply mains 

 in series with a non-inductive resist- 

 ance R. The total current i which 



delivered to the receiving circuit flows through the fixed coil B, is 

 a current which is proportional to the supply voltage e flows 



