[Illustration: Fig. 559.--Leeds and Northrup portable galvanometer (pointer type A). The sensitiveness of this instrument is such that it may be substituted in numerous cases for the non-portable reflecting type of galvanometer; as for instance, in the checking of ammeters and voltmeters to an accuracy of .2% by the potentiometer method, and on almost all Wheatstone bridge measurements to commercial accuracies. A current of 2 micro-amperes will cause the pointer to move 1 mm. over the scale, that is, it has a sensibility of 500,000 ohms. The method of suspending the moving system is such as to practically eliminate initial friction which is of value in all zero deflecting methods. The suspensions and moving system are guarded by springs, which together with the solid construction of the case render the instrument capable of withstanding rough usage. Overall dimensions are 5-1/4" × 2-5/8" × 3-1/2"; weight about 3 pounds.] Ques. What may be substituted for the voltmeter? Ans. A high resistance galvanometer, whose deflections are proportional to the current, the value of the deflections being substituted in the formula. Ques. What precaution should be taken in making the test? 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. [Illustration: 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. [Illustration: 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.]