CHAPTER IV 

 THE MEASUREMENT OF RESISTANCE 



Volt and Ammeter Method. The most obvious method of 

 determining an electrical resistance is by the direct application of 

 Ohm's law. The potential difference between the terminals of 

 the resistor and the current flowing through it are measured by 

 appropriate instruments, which have previously been calibrated. 



It is important not to lose sight of the possible influence of the 

 measuring instruments on the results. With the terminal at 1 

 (Fig. 81) the voltmeter gives the proper potential difference, but 

 the ammeter measures the current through the unknown resist- 

 ance plus that through the voltmeter. If the resistance, X, be at 

 all comparable with that of the voltmeter, the error may be great 

 unless allowance be made for the voltmeter current. In this 



Fio. 81. Volt and ammeter method for measuring resistance. 



case the voltmeter resistance must be known. If the terminal be 

 at 2, the ammeter gives the proper current but the measured 

 potential difference includes the drop in the ammeter and its 

 connections; if this be an appreciable fraction of that in X, the 

 error will be large unless this drop is subtracted from the volt- 

 meter reading. These considerations should be given weight when 

 making connections for any particular test. 



If the voltmeter be of low resistance, care must be taken that 

 the resistances of the leads and contacts are negligible. The volt- 

 ammeter method finds frequent employment in emergency work 

 where comparatively rough measurements will suffice; for 

 instance, in determining armature resistance. 



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