[Illustration: Fig. 554.--Ordinary resistance box. It contains sets of standard resistances consisting of coils of insulated wire having low conductivity and small temperature coefficient. The ends of the coils are joined to the section of the bar between the plugs. The insertion of a plug cuts out a coil. In using, care should be taken to put the plugs in with a slight twist so that there shall be no resistance introduced by poor contact.] Fall of Potential Method.--This is a very simple method of measuring resistances, and one that is convenient for practical work in electrical stations because it requires only an ammeter, voltmeter, battery and switch--apparatus to be found in every station. The connections are made as shown in fig. 555. In making the test the ammeter and voltmeter readings are taken at the same time, and the unknown resistance calculated from Ohm's law. Accordingly, since: (1) amperes = volts / ohms solving for the resistance, (2) ohms = volts / amperes [Illustration: Fig. 555.--Fall of potential method of testing resistances; a convenient method for testing at stations, requiring only the usual instruments to be found at a station. The resistance of the voltmeter must be very high, otherwise the test must be made as in fig. 556.] EXAMPLE.--If in fig. 555 the readings show 6 volts and 2 amperes how many ohms is the resistance being tested? Substituting in formula (2) ohms = 6/2 = 3 Ques. Can this test be made with any kind of voltmeter? Ans. Its resistance must be very high to avoid error. When a voltmeter having small resistance is used, it should be connected so as to measure the fall of pressure across both ammeter and unknown resistance as shown in fig. 556. [Illustration: Fig. 556.--Fall of potential method of testing resistances; diagram showing connections for testing with low resistance voltmeter. The resistance measured with this connection will be the sum of the resistances of the coil and the ammeter. The resistance of the ammeter is usually known and can be subtracted from the sum to obtain the required resistance.] Differential Galvanometer Method.--This is what is known as a _nil_ or zero method, that is, a method of making electrical measurements in which comparison is made between two quantities by reducing one to equality with the other, the absence of deflection from zero of the instrument scale showing that the equality has been obtained. The test is made with a differential galvanometer, and resistance box connected as in fig. 557. The current then will divide so that part of it flows through the resistance being tested and around one set of coils of the galvanometer while the other part will flow through the resistance box and the other set of coils as indicated. When the resistance box has been so adjusted that its resistance is the same as the unknown resistance the current in the two branches will be equal, and the needle of the galvanometer will show _no deflection_.