[Illustration: Fig. 557.--Differential galvanometer method of testing resistances. In making the test, the resistance box is adjusted till the galvanometer needle shows no deflection. When this condition obtains, the resistance in circuit in the resistance box is equal to the unknown resistance, hence, a reading of the box gives the value of the unknown resistance.] Ques. What name is given to this method of testing? Ans. It is called a _zero_ method, distinguishing it from _deflection_ methods. Ques. For what kind of resistance is the method adapted? Ans. Since it is a nil or zero method, it is better adapted to the measurement of non-inductive than of inductive resistances. Ques. What precaution should be taken with inductive resistances? Ans. The current must be allowed to flow until it becomes steady to overcome the influence of self-induction. Ques. What may be said with respect to the differential galvanometer method? Ans. With an accurate instrument it is very reliable. [Illustration: Fig. 558.--Drop method of testing resistances. The apparatus is connected as shown and readings taken with voltmeter across known and unknown resistance. The unknown resistance is then easily calculated.] Drop Method.--This is a convenient method, and one which may be used for measuring either high or low resistances with precision. It is used for many practical measurements, and requires only a voltmeter, battery, known resistance and a two way switch. The instruments are connected as in fig. 558, and in making the test, the voltmeter is switched into circuit across the known resistance and then across the unknown resistance, readings being taken in each case. The value of the unknown resistance, is then easily calculated from the following proportion: drop across known resistance known resistance ------------------------------ = ------------------ drop across unknown resistance unknown resistance from which unknown known resistance × drop across unknown resistance resistance = --------------------------------------------------------- drop across known resistance