CHAPTER V 



31. General conditions to be satisfied by alternate-current instruments 32. Hot- 

 wire instruments 33. Electrostatic instruments for low voltages 34. Methods 

 of extending range of low-reading electrostatic voltmeters 35. High-voltage 

 electrostatic instruments 36. Wattmeters 37. Power factor indicators 

 38. Oscillographs. 



31. General Conditions to be satisfied by 

 Alternate -current Instruments 



SOME of the instruments employed in connection with continuous 

 currents are also available, without any modification, for use on 

 alternate-current circuits, while others would give totally different 

 readings on the two circuits. Permanent magnet instruments, which 

 for accuracy and convenience of use are unsurpassed so long as we 

 are dealing with continuous currents, are, of course, out of the question 

 for alternating-current measurements. 



The values of the p.d. and current with which we are concerned 

 are their r.m.s. values. It is, therefore, evident that if a measuring 

 instrument is to read correctly on both continuous and alternating- 

 current circuits i.e. if the readings are to be independent of fre- 

 quency the particular effect on which the reading of the instrument 

 depends must be proportional to the square of the current or p.d. to 

 be measured. 



Considering instruments intended to measure currents, we have 

 at our disposal several arrangements in which the force or couple, as 

 the case may be, acting on the movable part of the instrument is 

 determined by the square of the current.* The most important of 

 these arrangements are : (1) A very thin wire traversed by the 

 current, and fitted with suitable magnifying gear for indicating 

 the amount of elongation due to rise of temperature. The elonga- 

 tion depends solely on rise of temperature, and the latter is completely 

 determined if the mean value of the square of the current be given, 

 it being quite immaterial as to whether the current is alternating or 

 continuous. (2) An arrangement of two coils, with their planes 

 inclined at any suitable angle, connected in series with each other 



* And is independent of the Mnd of current used, i.e. whether alternating or 

 continuous. 



