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Millivoltmeter and shunt for measuring heavy currents. Leads are clamped to shunt potential terminals and their resistance and also the contact resistances where attached to the shunt enter into the calibration of the meter.
Simple potentiometer circuit. Standard cell has been omitted. Contacts at
The contact resistance between brass block
In measuring the contact resistance of binding posts the current passes through the screw and wire clamped under the post, thence through the body of the post. Potential points are placed, one near the bottom of the screw and one near the bottom of the hole in the body of the post. It is assumed that the contact resistance so measured is double the value to be found in the normal use of the post.
In measuring plug resistances the potential points are brought close to the plug surface at
These curves were the results of tests on the metal blocks shown in Fig. 3. The current was held steady at a known value and the fall of potential measured by a galvanometer. The blocks were both of brass.
Nine phosphor bronze laminations .4 mm thick not spaced apart
Type of brass or copper switch stud employed. A potential point was located just under the top surface of stud.
This stud differed from that shown in Fig. 12 in that it was of smaller diameter.
The above curves were obtained with switches of various types. The previous Figs. 7 to 13 inclusive give the principal dimensions of these switches and reference is made by letter to the corresponding curve in Fig. 14.
This conventional type of key had silver contacts about 2 mm in diameter. The potential points were determined by drilling very fine holes in the silver contacts near their interface and peening copper wires .1 mm diameter in the holes.
Contact resistance between two crossed platinum wires of No. 22 B. & S. Gage. One wire was a pointer on a galvanometer used as a relay and the other wire acted as a stop to close a circuit when pointer was sufficiently deflected.
Contact resistance between a standard taper plug and its block. In making the measurements plotted here the potential points were so located that two contacts were in series. See Fig. 5.
Table 1 CLASSIFICATION OF CONTACTS