54 ELECTRICAL MEASUREMENTS 



values of the multiplying power. Such a case arises when insula- 

 tion resistances are being measured. 



AMMETERS 



An ammeter, in distinction from a galvanometer, is an instru- 

 ment so constructed that the current strength in amperes can be 

 read directly. 



Before referring to various designs of these instruments, it 

 will be well to refer to certain considerations which have influ- 

 enced the development of indicating electrical instruments, 

 especially for direct-current work. 



1. The resistance of all current-measuring instruments should 

 be very low, while that of all instruments for measuring voltages 

 should be as high as practicable, the reason in both ca^es being 

 that the disturbance of the circuit conditions by the insertion of 

 the instrument must be reduced to a minimum. Another way of 

 stating the same thing is that the energy dissipated in the instru- 

 ment must be a minimum. 



2. The construction must be such that the instrument will 

 maintain its reliability. The relative positions of the parts must 

 be maintained in spite of rough handling and the strength of all 

 magnets used must be insured by proper ageing. 



3. There must be no "set" of the controlling springs due to 

 standing under load and no indefiniteness of the zero reading due 

 to magnetic impurities in the movable coils. 



4. The indications of the instrument must be independent of 

 stray fields. The importance of this in industrial testing cannot 

 be over-emphasized. 



5. The indications must be independent of room temperature 

 and no errors must result from the heating due to the passage 

 of the current. 



6. All shunts must be free from errors due to thermo-electro- 

 motive forces. 



7. Ammeter shunts must be so constructed that they will not 

 be injured by abnormal currents of short duration and ample 

 provision must be made for dissipating the heat due to continuous 

 operation. 



8. There must be no effects due to the retentiveness of any 



