606 



ELECTRICAL MEASUREMENTS 



It is important to take the zero reading, for it is subject to 

 variation; and if taken and separately allowed for, a calibration 

 may retain its value, if the scale be equally divided, even though 

 the zero reading alter. 



Ammeter Calibration. To calibrate an ammeter it is neces- 

 sary to have a method for measuring the current which will be 

 free from such instrumental errors as may affect the indications 

 of even the best direct-reading standard instruments. Such 

 calibrations may be carried out by the aid of standard cells and 

 accurately adjusted resistances, as follows. 



FIG. 370. Connections for ammeter calibration. 



Referring to Fig. 370, R is a known resistance so constructed 

 that it will not heat appreciably with the passage of the current. 

 This is inserted in series with the ammeter to be calibrated, A. 

 Rh is a rheostat for controlling the current, and B is a battery of 

 storage cells to furnish the steady current necessary in such 

 work. 



To measure the ammeter current it is necessary to determine 

 the P.D. between the potential terminals of R. This may be done 

 by the method of projection of potentials, an application of Pog- 

 gendorff's method (page 269). To apply this it is necessary to 

 have an auxiliary battery, BI, of constant e.m.f . and capable of 

 furnishing a small current, 0.001 amp., continuously; and two re- 

 sistance boxes, ri and r 2 , of a total resistance of about 10,000 ohms 



