[reilley] CONTACT RESISTANCE OF METALS AND ALLOYS 127 



The resistance coils of the galvanometer were equal to about 

 100 ohms. Care was also taken in the calibration to have the same 

 leads in the circuit in order that the total resistance of the various 

 parts of the circuit would be the same for the contact wires as when 

 the standard resistance box was substituted. 



(c) Details of contact formation. 



Contact between two pieces of the wire conductor was obtained 

 by arranging the wires at right angles to each other, one of which was 

 stationary, while the other one was movable. The former was held 

 firmly in a brass clamp, 4 cms. long, 1 cm. wide and ^-cms. in thick- 

 ness. The upper adjustable jaw of the clamp was 2 cms. long. Through 

 it passed two upright brass screws, in each of which was drilled a hole 

 to admit the end of the fixed or stationary wire. The upright screws 

 also served as terminals for the shunt leads, and when tightened 

 held the wire firmly. At right angles was another clamp to which was 

 attached the second wire of the contact. Considerable difficulty 

 was experienced in getting rid of the effects due to the elastic proper 

 ties of the various conductors. This was accomplished, however, 

 by the use of a flat hinged brass jaw, to which the wire could be 

 soldered. The jaw itself was 6-3 cms. long varying in width from 

 1-5 cms. at the hinge to -5 cms. at the smaller end. Through the 

 end of the heavy brass clamp was drilled a small hole to contain 

 the pivotal hinge; while to the other end was soldered a small brass 

 hook. This second clamp was similar to the one holding the stationary 

 wire in that it had two upright brass screws, one a terminal for the 

 shunt lead and the other a terminal for two small flexible copper strands 

 soldered to the hinged brass jaw to insure good electrical connection 

 between the clamp and the jaw. The two clamps with their attach- 

 ments were made fast to a wooden base so constructed that it could 

 be weighted to prevent shifting of its position. 



(d) Method of varying and measuring the contact pressure. 



The hooked end of the hinged jaw, referred to in the preceding 

 section, was attached to one end of the beam of a very sensitive 

 balance, and by merely applying pressure to the other pan of the balance 

 the contact pressure could be increased or decreased as desired. 

 A glass beaker containing water was used in producing the necessary 

 pressure by placing it on the balance pan. Water was added or 

 withdrawn in large or small quantities as the occasion demanded 

 to secure a change of pressure. 



