Fig. 590.--Diagram of the Queen standard potentiometer. The circuit arrangement is a method of sub-dividing the main potentiometer wire, MNOPQ, so as to provide for very accurate reading. The secondary voltage, or that used to supply current to the main potentiometer circuit, is adjusted by regulating rheostats, "Fast," "Medium," and "Slow" so that the current flow is exactly .0001 ampere. It is noted that this instrument requires a very small current for its operation. The instrument is direct reading for voltage measurements, not exceeding 1.4+. In order to determine if the current flow through the potentiometer be exactly .0001 ampere, the terminals of the standard cell binding posts are connected in circuit so that the drop over points between which they are connected are exactly equal to the voltage of the standard cell used. Binding posts are provided for connection with various standard cells. The unknown voltage to be measured is placed in opposition to the current flow in the potentiometer circuit by connecting to the binding post "XEMF." Observe that polarity is connected as required. The galvanometer with its shunt is placed in the standard cell circuit, or X circuit, by means of a double pole, double throw switch. The switch at T provides for standard cells of different values and the setting at U allows for temperature correction. The range of the instrument in volts can be increased by means of multipliers or volt boxes.
Figs. 591 to 594.--Diagrams illustrating loop testing. To properly understand the Murray or Varley loop tests, consider a Wheatstone bridge ([fig. 591]) the arms of which are equal. In loop testing, the rheostat is replaced by a length of cable and the unknown resistance also by a length of cable, as in [fig. 592], both being similar in resistance per foot. If both lengths be the same, their resistances are the same and the bridge balances. Now shorten one cable and add resistance in series with it until the bridge again balances as in [fig. 593]. The added resistance equals that of the piece cut off. Hence, if the resistance per foot be known, the length of the shorter piece can be easily calculated. In the Murray and Varley tests, the battery circuit is by ground connections instead of by wire. In the Murray loop the arrangement is similar to [fig. 592], the battery circuit being completed by ground connection through fault in defective cable. [Fig. 594] shows the general arrangement of the Varley loop.