Simpson true RMS Voltmeter


Simpson Electric Company Chicago Illinois USA True RMS Decading Voltmeter model 1901 Serial no. 1030. 1000 ohms per volt, 0-20 kHz

This instrument is housed in a 430mm X 230mm wood effect laminate box with carrying handle. I have no information on this piece of equipment and after some careful experimentation have discovered that it needs a 45 volt battery to provide the calibration potential to adjust the meter needle to zero prior to measuring the unknown voltage by adjusting the decade resistance knobs to again obtain a zero reading. I have made up a 45 volt supply using five PP3 batteries wired in series.

Inside view

Electrothermic transfer standards for current and voltage use a thermo element consisting of a heater and a thermocouple. The heater is a short straight wire suspended by two supporting lead-in wires in an evacuated glass bulb (see picture below) At the mid point of the heater wire and electrically isolated from it is a thermocouple encased in a minute bead. The thermal emf ( a few millivolts) generated by the thermocouple is a measure of the heater current.

Electrothermic element