Standard Cells for 100 Years

The US national volt was maintained by standard cells (batteries) for many years. English engineer Josiah Latimer Clark invented the Clark cell in 1873. It is a wet chemical cell that produces a highly stable EMF (voltage) suitable for use as a voltage standard (shown below).

The Clark cell produced a voltage of 1.4328V at 15º C (288 kelvin). It had a rather large temperature coefficient (TC) of -1.15 mV/ºC. The Clark cell was legalized as the standard of EMF in the US in 1894 and in several other countries about that time. During the years 1893 to 1905 the cell devised by Edward Weston was found to have many advantages over it, and was officially adopted in 1908 at the London International Conference of Electrical Units and Standards according to NBS monograph 84. Some more early history of standard cells and information can be found here. The Weston (or cadmium sulfide) cell is shown below.

It has an emf TC about 1/30^th of the Clark cell, better emf stability, and an emf closer to 1V. The voltage depends on the concentration and acidity of the solution, the composition of the amalgam, the crystal phases of cadmium sulfate and the temperature and pressure. New unsaturated Weston cells generally range in emf from 1.0190 to 1.0194 V and on the average reach 1.0183 V within 23 to 37 years, providing they are maintained at about 25ºC and not subjected to abuse. When they reach 1.0183 V they generally behave erratically.^[1]

Standard cells at the National Bureau of Standards (now called the National Institute of Standards and Technology)(NIST) were kept in oil baths to maintain constant temperature, thereby reducing the problem of temperature variations to a negligible amount. They kept the temperature variation of the cells to less than 0.002ºC per day. Primary standards labs had their cells in a temperature controlled air bath enclosure in groups of 4 or 6. It was wise to have two baths, one kept at the lab to use while the other was sent for calibration to NIST. For best results, an inter-comparison was done before the cells left, and after they came back. This gave evidence for changes during shipping.

Originally the comparison was done in the lab where I worked with a Guildline 9930 Direct Current Comparator Potentiometer. It had an optical readout galvanometer. It was essentially a dc transformer. It used magnetic field canceling by changing current somewhere (possibly the galvo itself). A standard cell that was picked to use as the standard was connected to one side, and a set of dials was adjusted to its voltage. The cell under test was connected to some other terminals, and another set of dials was adjusted until the galvo came to a null. The unknown's voltage was read from the second set of dials. This may be oversimplified (I'm pushing my memory). This process was repeated until sufficient comparisons were made.

Later a standard cell scanner was used to compare the cells (in conjunction with a digital voltmeter). Comparisons were done in accordance with a statistically derived experiment.. Some of our cells had some small radioactivity. A cells voltage would suddenly increase 1 to 2 ppm for a second or two, then return to its previous value. I assume it was when an atom emitted a particle. That complicated the comparison process. Our lab was able to maintain the dc volt to an uncertainty of 1-2 parts per million (ppm) with standard cells.

Although I haven't seen one in about 20 years, they are still sold on ebay. It's been over 100 years since they were adopted. I doubt that any serious calibration lab is still using them. Zener references have made them obsolete. Zener references may be the subject of my next thread.

-S

[1] Precision Measurement and Calibration Electricity - Low Frequency (NBS Special Publication 300 - Volume 3 Dec. 1968)