Direct-voltage references

Precise sources of e.m.f have existed mainly in three forms: the electrolytic cell, the gas-discharge tube and the avalanche or Zener diode. The paper attempts to place these various voltage-reference devices in historical and technical perspective. The properties of several types of cell are given, followed by a similar treatment of glow-discharge and corona-discharge tubes and finally a rather fuller account of the Zener diode and other solid-state elements. The relative merits of all these reference devices are then discussed. The paper concludes with an outline of some of the latest techniques for the successful use of these devices. The list of papers to which reference is made includes most of the original texts.