The potential difference of the ends of the low resistance is at the same time measured on the potentiometer, and the quotient of this potential difference by the known value of the low resistance gives the true value of the current passing through the ammeter.
Hence if the galvanometer is calibrated by a potentiometer we can determine the value of this current in amperes, and knowing the value of n and V thus determine C. Various forms of commutator have been devised for effecting this charge and discharge rapidly by J.
POTENTIOMETER, an instrument for the measurement of electromotive force and also of difference of electric potential between two points.
The term potentiometer is usually applied to an instrument for the measurement of steady or continuous potential difference between two points in terms of the potential difference of the terminals of a standard voltaic cell of some kind, such as a Clark or Weston cell.
The modern potentiometer has been developed out of an arrangement due to J.
In principle the modern potentiometer consists of an arrangement by means of which any potential difference not exceeding a certain assigned value can be compared with that of a standard cell having a known electromotive force.
1), and the potentiometer wire itself has a resistance equal to one of these coils.
One terminal of the galvanometer can then be shifted to the junction 6 7 g between any pair of consecutive coils and the slider shifted to any point on the potentiometer wire.
In some cases the potentiometer wire is wholly replaced by a series of coils divided into small subdivisions.
We may employ such a potentiometer to measure large potential difference greater than the electromotive force of the working battery, as follows: The two points between which the potential difference is required are connected by high resistance, say of 100,000 ohms or more, and from the extremities of a known fraction of this resistance, say, 'Roo or I/1000 or I/Io,000 wires are brought to the potentiometer and connected in between the slider and the corresponding galvanometer terminal.
The potentiometer and the divided resistance constitute a sort of electrical scaleyard by means of which any electromotive force or difference of potential can be compared with the electromotive force of a standard cell.
C, The set of equal potentiometer coils in series with it.
From the potential terminals of the strip, wires are brought to the potentiometer so as to determine their potential difference in terms of the electromotive force of the standard Clark cell.
In the same manner the potentiometer may be used to calibrate a voltmeter by the aid of a divided resistance of known value.
In electrical measurements connected with incandescent electric lamps the potentiometer is of great use, as it enables us to make accurately and nearly simultaneously two measurements, one of the current through the lamp and the other of the potential difference of the terminals.
For this purpose a resistance, say, of one ohm is placed in series with the lamp and a resistance of 100,000 ohms placed across the terminals of the lamp; the latter resistance is divided into two parts, one consisting of loon ohms and the other of 99,000 ohms. The potentiometer enables us to measure therefore the current through the lamp by measuring the drop in volts down a resistance in series with it and the potential difference of the terminals of the lamp by measuring the drop in volts down the tooth part of the high resistance of 100,000 ohms connected across the terminals of the lamp.
A necessary adjunct to the potentiometer is some form of standard cell to be used as a standard of electromotive force.
C. Fisher, The Potentiometer and its Adjuncts (London, 1906).
The Difference Of Potential E Between The Ends Of The Tube, And The Electric Current C Through It, Are Measured On An Accurately Calibrated Potentiometer, In Terms Of A Clark Cell And A Standard Resistance.
The principles of telegraphy (land, submarine and wireless) and of telephony are discussed in the articles Telegraph and Telephone, and various electrical instruments are treated in separate articles such as Amperemeter; Electrometer; Galvanometer; Voltmeter; Wheatstone'S Bridge; Potentiometer; Meter, Electric; Electrophorus; Leyden Jar; &C.
Later observers have generally employed a balance method (some modification of the potentiometer or Poggendorf balance) for measuring the E.M.F.
The electromotive force so selected is balanced against the steady potential difference produced between a fixed and a sliding contact on a wire traversed by another steady current, and if there is any difference between this last, the potential difference, and the instantaneous potential difference balanced against it, a relay is operated and sets in action a motor which shifts the contact point along the potentiometer wire and so restores the balance.