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Standards

Everywhere a measurement is made, the subject of that measurement is being compared to some kind of a standard. After all, a volt is a volt, no matter who is doing the measuring. But how did laboratories in different places, at different times, ensure that they all had the same size of unit?

Weston cell

Weston standard cells

Weston standard cells

Inside Weston standard cell.

Inside Weston standard cell.

For much of the 20th century one of the most convenient standards for electrical potential was the Weston cell, a specific kind of primary cell that delivers a very stable voltage under known conditions, nominally 1.018638V if the cell is chemically saturated. To maintain accuracy, the Weston cell must be used at the correct temperature and connected in such a way that no current is normally drawn from it. Precautions included enclosing the cell in an oil bath closely monitored with a thermometer, and connecting it in a bridge circuit that passes current through the galvo only momentarily until balanced.

Standard resistance

Tinsley standard resistance

Tinsley standard resistance

Inside Tinsley standard resistance

Inside Tinsley standard resistance

Whilst the unit of resistance, the ohm, is defined in terms of other quanitites, it is often convenient to have a physical device that implements that definition in a sufficiently accurate way for the task in hand. I.e, some kind of resistor that is an adequate representation of the ohm. This 100 milliohm standard resistor is made in such a way as to maximise stability over time, temperature and current. Although the resistance wire consists of an alloy with low temperature coefficient of resistance, further steps are taken to ensure that the entire wire is held at a known temperature. The canister is filled with oil, the temperature of which can be monitored with a thermometer inserted through the cover. To prevent the heat dissipated by the resistance causing local warming of the oil around the wire, a mixing paddle is attached to the inner end of a rod that can be used to agitate the oil prior to reading the thermometer. Special ‘4-wire’ terminals are provided to avoid the resistance of external connections being included in the measurement. Separate voltage and current leadouts allow the maker to define the exact point along the current circuit at which the calibrated resistance begins.

Tuning forks

Frequency (and its reciprocal, period) can be referenced to the time constant of an electrical circuit, however it is difficult to maintain a very high degree of stability using conventional components such as inductors and capacitors. Over the last few decades, the quartz oscillator has become the ubiquitous frequency standard for most applications from radio transmitters to wristwatches, yet the quartz crystal itself is tuned to a mechanical resonance, not an electrical one. Before the advent of quartz oscillators, another mechanical resonant system was used to provide a frequency reference - the humble tuning fork. A pair of contacts can be arranged to interrupt the current in a coil that attracts the steel fork magnetically, making the electrically driven fork a close relative of the electric bell. In the smaller 100Hz fork by Tatlock below, the coil drives one tine while the other operates the contact. The Tinsley 50Hz fork uses two pairs of contacts, one to interrupt the exciting coil current and another to transmit the 50Hz frequency reference to the external circuit.

100Hz electric tuning fork

100Hz electric tuning fork

Tuning fork coil

Tuning fork coil

Tinsley electric tuning fork

Tinsley electric tuning fork


Sub-standards and working standards

Much engineering and lab work is carried out to a specified accuracy, very often much lower than theoretically be achieved using the best available standard. There is little point in using the delicate and fiddly Weston cell to calibrate a circuit that includes other components with no better than 10% accuracy, for example. Therefore it is convenient to calibrate various sub-standards and working standards for everyday use, such as the popular resistance boxes shown below. These are inferior to the standard resistance with regard to time and temperature stability, their initial calibration includes any error from the original standard plus an error margin for the calibration procedure, yet they are much more versatile for basic work around the lab.

Harris resistance box

Harris resistance box

Cambridge decade resistance

Cambridge decade resistance

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