Et = E20 - 0.0000406(t-20) - 0.00000095(t-20)2 + 0.00000001(t-20)3
The Weston standard cell, or unsaturated form is practically unaffected by changes in temperature and is the form most commonly used for laboratory work and general testing. The average pressure of this form is 1.0187 Int. volts.
Ques. Define the International volt.
Ans. It is the electromotive force that, steadily applied to a conductor whose resistance is one International ohm, will produce a current of one International ampere, and which is represented sufficiently well for practical use by 1,000/1,434 of the voltage between the poles of the Clark cell at a temperature of 15° C., when prepared as in [fig. 540].
The relation between the units volt, ampere and ohm, are shown graphically in figs. [542] and [543].
Figs. 542 and 543.--Diagrams showing hydraulic analogy illustrating the difference between amperes and coulombs. The rate of current flow of one ampere in [fig. 543] may be compared to the rate of discharge of a pump as in [fig. 542]. Assuming the pump to be of such size that it discharges a gallon per stroke and is making 60 strokes per minute, the quantity of water discharged per hour (coulombs in [fig. 543]) is 1 × 60 × 60 = 3,600 gallons. Following the analogy further (in [fig. 543]), the pressure of one volt is required to force the electricity through the resistance of one ohm between the terminals A and B. In [fig. 542], the boiler must furnish steam pressure on the pump piston to overcome the friction (resistance) offered by the pipe and raise the water from the lower level A' to the higher level B'. The difference of pressure between A and B in the electric circuit corresponds to the difference of pressure between A' and B'. The cell furnishes the energy to move the current by maintaining a difference of pressure at its terminals C and D; similarly, the boiler furnishes energy to raise the water by maintaining a difference of pressure between the steam pipe C and exhaust pipe D'.
Fig. 543. If the current strength in [fig. 543] be one ampere, the quantity of electricity passing any point in the circuit per hour is 1 × 60 × 60 = 3,600 coulombs.
Current Measurement.--It is necessary to adopt some arbitrary standard in order to compare currents of different strengths. The term strength of a current, or current strength means the rate of flow past any point in the circuit in a given unit of time. The unit of current, called the ampere, is defined as the unvarying current which, when passed through a solution of nitrate of silver in water (15 per cent. by weight of the nitrate) deposits silver at the rate of .001118 gramme per second.