|Voltage Outputs||1.0V, 1.018V, 1.019V, 0-1mV|
The 735A is a compact DC voltage transfer standard. It has a single output whose voltage is selectable via a front-panel rotary switch. The output choices are 1.0V, 1.018V, 1.019V, and adjustable 0V-1mV. The output is floating, but both ground and guard terminals are available on the front panel.
A multi-turn variable resistor with a turns counter dial sets the output voltage between 0V and 1mV in the variable setting. The 1.018V and 1.019V output settings are base voltages to which a variable positive delta is added using the same front panel control.
The 1.018V and 1.019V settings may seem peculiar today. These are the voltages of saturated and unsaturated Weston Standard Cells, which are a type of electrochemical voltage reference that was very common up until a few years ago. When properly constructed and handled, the voltage generated by such a cell is precisely calculable and very stable. Large banks of these cells were used with statistical monitoring and averaging processes by the US NBS (now NIST) to maintain the US Standard Volt for many decades. Standard cell voltages are actually a bit higher than 1.018V and 1.019V, hence the positive delta setting. This allows matching of the 735A's output to that of a standard cell (or a bank of them) to transfer its voltage to another instrument or site.
The specified stability is <10ppm/month and the specified transfer accuracty is 2ppm, both after a 30-minute warm-up period.
As with most such voltage references, the heart of the 735A is a special Zener diode enclosed in a thermostatically-controlled oven. An amplifier and precision voltage dividers then buffer and scale the output as required.