Electrical measurementsA laboratory manual . lumn of Deflections in millimetres, the larger num-bers are the products of the deflections and the multiplying powerof the shunt. 60. Insulation Resistance by Leakage. — The method consists in charging the cable as a condenser,letting it leak for a few observed seconds, and thencharging to the full potential again by connectingthrough the galvanometer. ^Electrical Engineer, May 20, 1891, p. 565. 88 ELECTRICAL ME A S UBEMEN TS. First. To find the constant of the ballistic galva-nometer G- (Art. 97). This may be done in two first consists in
Electrical measurementsA laboratory manual . lumn of Deflections in millimetres, the larger num-bers are the products of the deflections and the multiplying powerof the shunt. 60. Insulation Resistance by Leakage. — The method consists in charging the cable as a condenser,letting it leak for a few observed seconds, and thencharging to the full potential again by connectingthrough the galvanometer. ^Electrical Engineer, May 20, 1891, p. 565. 88 ELECTRICAL ME A S UBEMEN TS. First. To find the constant of the ballistic galva-nometer G- (Art. 97). This may be done in two first consists in charging a condenser of knoAvncapacity by a known , and then discharguigthrough the galvanometer. Let the apparatus be set upas shown in Fig. 38, in which ^ is a charge and dis-charge key, C isthe condenser,and Gr the galva-nometer. Thebattery B maybe a standardcell, the which isknown. Then ifQ is the quantityof electricity dis-charged throughthe galvanom-eter, O the capacity of the condenser, and ^i the the Fig. 38. If the deflection is di, and k = ^1 The other method^ involves the exact measurementof a current. A long magnetizing coil is uniformlywound on a wooden cylinder or other non-metallic core,the diameter of which is accurately known. Over this ^ Ewiags Magnetic Induction in Iron and other Metals^ p. 62. BESISTANCE. 89 primary, at the middle of its length, a short secondarycoil is AYOund and put in circuit with a ballistic galva-nometer. Let A be the mean area of cross-section of the primarycoil, and let ^i be the number of turns in it per Then if a current of /amperes be made to passthrough the coil, the magnetic flux or induction within it near the middle is per square centimetre,^ and the total number of lines of induction within the coil is4:7rInA 10 If iV is the number of turns in the secondary and rthe resistance in the circuit of the galvanometer, thenthe quantity of electricity in coulombs passing dur-ing the flow of
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