Ans. The current used should not be strong enough to appreciably heat the resistance, and if the current be not very steady, several readings should be taken of each measurement and the average values used in the formula.
Ques. How are the most accurate results obtained?
Ans. By selecting the known resistance as near as possible to the supposed value of the unknown resistance.
Fig. 560.--Voltmeter method of testing resistances. Knowing the resistance of the voltmeter, turn switch to the left and from reading calculate resistance corresponding to one division of the scale. Turn switch to right and multiply reading by resistance required for deflection of one division. This gives resistance of voltmeter and unknown resistance; subtracting from this the resistance of voltmeter gives value of the unknown resistance.
Voltmeter Method.--This is a direct deflection method and consists in determining first the resistance that will deflect the needle through one division of the scale on a given battery current, then with this as a basis for comparison the voltmeter is connected across the unknown resistance whose value is easily calculated from the reading.
In making the test, the instruments are connected as in [fig. 560]. The current from battery is first passed through the galvanometer by turning switch as shown.
Fig. 561.--Megohm box or set of standard high resistances. The box contains five resistances of 200,000 ohms each. The six pillars are petticoat insulated, the resistances being placed between each pair of pillars. There is a double contact post on top of each pillar so that these can be connected together with copper links.
Assuming that the resistance of the instrument is 8,000 ohms and that the current deflects the needle through 10 divisions of the scale, then for a deflection of one division the resistance is
8,000 × 10 = 80,000 ohms.
Accordingly, if, when the switch is moved to the right, connecting the voltmeter across the unknown resistance, the needle be moved through 6 divisions of the scale, the combined resistance of the voltmeter and unknown resistance is
80,000 ÷ 6 = 13,333-1/3 ohms,
and subtracting the resistance of the voltmeter, the value of the unknown resistance is
13,333-1/3 - 8,000 = 5,333-1/3 ohms.