Drop Method.--This is a convenient method, and one which may be used for measuring either high or low resistances with precision. It is used for many practical measurements, and requires only a voltmeter, battery, known resistance and a two way switch.
The instruments are connected as in [fig. 558], and in making the test, the voltmeter is switched into circuit across the known resistance and then across the unknown resistance, readings being taken in each case. The value of the unknown resistance, is then easily calculated from the following proportion:
| drop across known resistancedrop across unknown resistance | = | known resistanceunknown resistance |
from which
| unknown resistance | = | known resistance × drop across unknown resistancedrop across known resistance |
Fig. 559.--Leeds and Northrup portable galvanometer (pointer type A). The sensitiveness of this instrument is such that it may be substituted in numerous cases for the non-portable reflecting type of galvanometer; as for instance, in the checking of ammeters and voltmeters to an accuracy of .2% by the potentiometer method, and on almost all Wheatstone bridge measurements to commercial accuracies. A current of 2 micro-amperes will cause the pointer to move 1 mm. over the scale, that is, it has a sensibility of 500,000 ohms. The method of suspending the moving system is such as to practically eliminate initial friction which is of value in all zero deflecting methods. The suspensions and moving system are guarded by springs, which together with the solid construction of the case render the instrument capable of withstanding rough usage. Overall dimensions are 5-1/4" x 2-5/8" x 3-1/2"; weight about 3 pounds.
Ques. What may be substituted for the voltmeter?
Ans. A high resistance galvanometer, whose deflections are proportional to the current, the value of the deflections being substituted in the formula.
Ques. What precaution should be taken in making the test?