THE CALIBRATION OF INSTRUMENTS 005 



B is a battery capable of giving the desired current; W is a 

 water rheostat by which the current I v , and consequently the 

 reading of the voltmeter F, may be varied; r is a known and 

 variable resistance. The standard cell has a voltage denoted by 

 E. One must be sure that the cell is properly inserted so that in 

 the galvanometer circuit its e.m.f. will oppose the P.O. due to the 

 drop in r. If the cell be so inserted and I v r = E, the galva- 

 nometer will remain undeflected when the key K is depressed 



7 - E 

 Iv-~ 



ancl the P.D. across the terminals of the voltmeter will be given by 



P.D. =~R V . 



In practice it is usually desired to calibrate at or near certain 

 predetermined points, at readings of 10, 20, 30 volts, and so on. 

 It is, therefore, necessary to know the value of r which must be 

 inserted in order that when the galvanometer is balanced the 

 voltmeter reading may be that desired. This is readily de- 

 termined, for suppose that the instrument is to be calibrated 

 at or near a reading of 30 volts, that E = 1.0186 and R v = 

 17,000 ohms. The proper value of r would be, 



1.0186 X 17,000 



r = - on - = 577 ohms. 



oU 



577 ohms are to be inserted at r, and by the water rheostat, W, 

 the reading of the voltmeter is to be brought to 30 volts. It 

 may be necessary to change the number of battery cells. The 

 key K should be depressed cautiously, and released immediately 

 the deflection appears. In general there will be a deflection, for 

 the voltmeter will probably have a slight error, so that although 

 it reads 30 volts, the true P.D. between the terminals will differ 

 from that value; also, the resistance r is not exactly the value 

 corresponding to 30 volts for r is adjustable to single ohms only. 

 An exact balance is obtained by varying the water rheostat, 

 and the voltmeter is read immediately thereafter. The reading 

 and the corresponding value of r must be recorded. The difference 

 between the true P.D. at the instrument terminals and the 

 reading gives the correction, the quantity which must be added 

 to the reading to obtain the true P.D. 



