The apparatus required consists of battery, galvanometer, known resistance, and double contact key. The connections are made as in fig. 550. The known resistance is put in circuit with the galvanometer and after noting the deflection, the key is moved so as to cut out the known resistance and throw into circuit the unknown resistance. The deflection of the galvanometer is again noted and compared with the first deflection. [Illustration: Fig. 551.--Charge and discharge key, adapted to condenser testing where the condenser is to be alternately charged and discharged. The insulated handle enables the key to be used without insulating the body.] [Illustration: Fig. 552.--Pohl commutator. This is equivalent to a two pole double throw switch. The depressions in the base are filled with mercury into which the contacts dip in closing the circuit.] If the deflections be proportional to the current, the unknown resistance will be as many times the known resistance as the deflection with the known resistance is greater than the deflection with the unknown resistance. Method of Substitution.--This is the simplest method of measuring resistance. The resistance to be measured is inserted in series with a galvanometer and some constant source of current, and the galvanometer deflection noted. A known adjustable resistance is then substituted for the unknown and adjusted till the same deflection is again obtained. The value of the adjustable resistance thus obtained is equal to that of the resistance being tested. [Illustration: Fig. 553.--Substitution method of testing resistances. The connections and apparatus are the same as in fig. 550, except that a resistance box is used in place of the known resistance. In making the test, first note deflection with unknown resistance in circuit, then press key so that the current will pass through the resistance box, and adjust the resistance in the box so that the deflection of the galvanometer is about the same as with the unknown. Now switch from one circuit to the other, changing the resistance in the box until equal deflections are obtained. When this obtains, the resistance in the box is the same as the resistance being tested.] Ques. What kind of adjustable resistance is used in making the above test? Ans. A resistance box. Ques. Describe a resistance box. Ans. It consists of a box containing numerous resistance coils with their ends connected to terminals and provided with plugs so that they may be thrown into or out of circuit at will, thus varying the resistance in the circuit.