58 ELECTRICAL MEASUREMENTS 



terminal blocks which can be interleaved with the busbar. The 

 heat is thus disposed of by conduction as well as by convection. 



Reference should be had to the introduction to the chapter on 

 the " Calibration of Instruments" where various errors found in 

 commercial ammeters, voltmeters, etc., are discussed. 



Thermo-ammeters. The rise of temperature of a wire carrying 

 a current is a function of the current strength; it may be utilized 

 for purposes of measurement. Various forms of indicators have 

 been devised. They utilize the change of electrical resistance of 

 the wire, its expansion, or its rise of temperature. 



For alternating-current work the hot-wire or thermal principle 

 possesses certain theoretical advantages, due to the fact that 

 there are neither coils nor soft iron in the instruments; hence, 

 inductance effects are reduced to a minimum and saturation 

 effects eliminated altogether. With proper design such instru- 

 ments should then be applicable to both direct- and alternating- 

 current circuits and their indications should be independent of 

 frequency, wave form, tnd stray-field effects. 



The practical difficulties met with are due to the uncertainty 

 of the zero reading, to the sluggishness of action due to the heat 

 capacity of the various parts of the instrument and to the influ- 

 ence of room temperature. Also, as the carrying capacity of 

 the hot wire is seriously taxed, there is the liability of burning 

 out the instrument by a temporary overload or short-circuit, 

 which in the ordinary type of instrument would result in nothing 

 worse than a bent pointer. Usually the energy consumption in 

 this class of instruments is large. 



All things considered, hot-wire instruments are unsuited for 

 switchboard work. Their particular field of usefulness is in high- 

 frequency work, such as radio-telegraphy or in the laboratory 

 where they are employed as "crossover" instruments between 

 alternating and direct currents in calibration work. 



At ordinary frequencies shunts may be employed, but for 

 switchboard work no great advantage results from this, for in 

 American practice all alternating-current instruments on circuits 

 of above 500 volts are actuated through transformers, thus keep- 

 ing the front of the switchboard free from high-voltage circuits 

 which would be a source of danger. 



The earliest commercial instrument based on the thermal or 



