138 PHYSIOLOGICAL PHYSICS. [Chap. xin. 



offered by a conductor is dependent on the tempera- 

 ture. Increased temperature diminishes resistance, 

 that is, increases conductivity. 



To measure resistances the simple rheostat 

 may be used. A Daniell's element is placed in the 

 circuit of a galvanometer ; an extremely sensitive one 

 is not necessary. When the current is permitted to 

 pass through the galvanometer the needle is deflected. 

 If, now, the body whose resistance is to be measured 

 be placed in the same circuit, the current from the 

 Daniell, encountering the resistance of the body, will 

 not be able to deflect the needle to the same extent. 

 Note the amount of deflection. Remove the body, and 

 place, instead, the rheostat in circuit. Send the cur- 

 rent again through the galvanometer, and then wind 

 the rheostat wire on to the wooden cylinder ; this 

 adds resistance, and the needle will be brought slowly 

 back from its maximum deflection caused by the 

 Daniell. Wind on the wire till the resistance it offers 

 causes the needle to be brought back to the position 

 it occupied when the body to be measured was in 

 circuit. At that position the amount of wire wound 

 on to the wooden cylinder will be the measure of the 

 resistance offered by the body. 



A method of greater value is the method by 



WHEATSTONE'S BRIDGE. It 

 shown in diagram in 

 71. 



At e is a Daniell's ele- 

 ment; and AB is the platinum 

 e wire of a long compensator, 



Fig 71. Wheatstoue's Bridge. * i r xi v J 



of which s is the slider. 



(See page 121.) From e the positive electrode goes to 

 A, the negative to B. From A a wire passes to r, the 

 body whose resistance is to measured, from which 

 again a wire is carried to one binding screw of Re, a 

 resistance box, from whose other binding screw a wire 



