700 



ELECTRICAL MEASUREMENTS 



! 



"plate" consisted of three sections, as shown, having a total 

 length of 240 cm. and a diameter of 162.8 cm. The central, or 

 working section, was 47.7 cm. long, and its capacity was calcu- 

 lated and found to be 2.657 X 1Q- 11 farads. 



The commutator may be driven by a synchronous motor. 

 The brushes must then be set so that the deflection of the galva- 

 nometer is a maximum. If an induction motor be used, the 

 deflection goes through a regular cycle, corresponding to the wave 

 form of the voltage applied to the condenser. The maximum 

 deflection of the galvanometer may then be read. 



Whitehead and Gorton in their researches on the dielectric 

 strength of air replaced the commutator by an arrangement of 

 mercury-arc rectifiers as shown in Fig. 440. 



Tran*. 



FIG. 440. Whitehead and Gorton arrangement of electrical valves for 

 measuring peak -voltage. 



A mercury arc with a mercury and an iron electrode allows 

 the current to flow from the iron to the mercury but effectually 

 prevents any flow in the opposite direction. 



To maintain the ionization in the tubes independently of the 

 small current whose mean value is to be measured, two sources 

 of direct current are used. 



The average value of the current is found by multiplying the 

 reading of the ammeter A by 2. 



The use of electrical valves has been further developed by 

 Chubb in the switchboard apparatus shown in Fig. 441. The 

 valves for suppressing alternate half-waves are placed in the 

 drawers marked "right" and "left" and are seen at V\ and F 2 in 

 the diagram. The anodes are of tungsten or molybdenum, the 

 cathodes of incandescent tungsten, the bulbs being filled with 

 mercury vapor. The cathodes are heated by alternating currents 

 supplied through two small transformers, B\ and B 2 . 



