598 ELECTRICAL MEASUREMENTS 



Balancing. Accurate balancing of the movable system is 

 essential, for commercial instruments should not require careful 

 levelling. As a test the instrument should be tilted from its 

 normal position in various directions and the pointer observed. 

 If lack of balance is found to be present and the instrument must 

 be used, it should be set up, using a level, the same precaution 

 being taken during the calibration. The rebalancing should be 

 done by an experienced person. 



Scale Errors. The cardinal points on the scale are supposed 

 to be laid off, for each particular instrument, by comparison with 

 a standard. However, the subdivisions are frequently very 

 carelessly made and their irregularities are often apparent on 

 inspection. The calibration curves for such irregular scales are 

 "lumpy" and the calibration points must be taken near together. 



Corrosion. Hard-rubber covers and instrument bases which 

 are imperfectly vulcanized may give trouble. The free sulphur 

 attacks delicate wires, controlling springs, and suspensions, 

 causing gradual deterioration and finally total failure. The 

 effect on the indications of the instrument is, of course, pro- 

 gressive. Fine wires insulated with soft-rubber tubes, as is 

 common for internal connections, may suffer in the same way. 



ELECTRICAL AND MAGNETIC ERRORS 



Shunts. Care must be taken that shunts suffer no mechanical 

 injury. In making connections for a test they should be firmly 

 bolted into the circuit, all contacts being clean. Imperfect 

 contact at one end of the shunt may result in unequal heating 

 and a consequent thermo-electromotive-force error; but the over- 

 zealous application of the monkey wrench should be avoided, for 

 if the shunt is not properly supported some of the soldered joints 

 where the resistance strips are sweated into the terminal blocks 

 may be broken, and though no damage is visible the shunt may 

 be rendered entirely untrustworthy and the test useless. The 

 current leads should be of ample size, so as to assist rather than 

 to hinder the dissipation of the heat from the shunt. During 

 calibrations, especially where high-capacity shunts are involved, 

 current connections identical with those of regular service must 

 be used, so that the current may be properly distributed before 

 the potential terminals are reached. 



