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The Time Electronics Voltage Source and Potentiometer type 404N, pictured right, is a 0.05% voltage source settable up to 0.9999V in 100 microvolt steps, to 0.09999V in 10 microvolt steps and to 0.009999 volts in 1 microvolt steps. These levels of precision are also available in the potentiometer mode. The instrument is particularly useful in calibrating other instruments and for measuring temperatures using thermocouples. With a cold junction at 0 degrees C, a type K thermocouple can be read to 0.025 deg C.
In a simple calibration experiment, a table of the setting of the Time Electronics 404N against voltage read by the Solartron Multimeter is shown below:
| setting | 999.9 range | 99.99 range | 9.999 range |
| 0.0000 | 0.000002 | 0.000040 | 0.000012 |
| 0.1000 | 0.100002 | 0.010005 | 0.001011 |
| 0.2000 | 0.200018 | 0.020014 | 0.002012 |
| 0.3000 | 0.300050 | 0.030013 | 0.003012 |
| 0.4000 | 0.400090 | 0.040017 | 0.004012 |
| 0.5000 | 0.500120 | 0.050024 | 0.005012 |
| 0.6000 | 0.600120 | 0.060024 | 0.006013 |
| 0.7000 | 0.700150 | 0.070028 | 0.007013 |
| 0.8000 | 0.800150 | 0.080029 | 0.008013 |
| 0.9000 | 0.900210 | 0.090037 | 0.009013 |
The accuracy of the 404N 0.05% model is quoted as "+/- 0.05% of setting, +/- 0.02% range." These figures have to be taken together and suggest that for part of the range of operation, the above instrument was inside its specification, except at the very highest and at the lower voltage settings. The manual for the Time Electronics 404N can be downloaded here.
Where voltages above 1 volt have to measured, two Time Electronics 404N Potentiometers can be used and configured as in the sketch on the right. Here the instruments are being used to measure the EMF of a Weston Cadmium cell. The left hand instrument is set as a voltage source and set to provide a potential of 0.9999 volts. The right hand instrument is configured as a potentiometer and set to the 99.99mV range at about 20 mV initially before the standard cell is connected. This procedure avoids large currents being drawn from the cell. After connection, the setting of the right hand instrument is adjusted to secure a null. The 1 microvolt sensitivity of the Time 404N enabled the short term variation of the voltage of the cell to be observed, which amounted to several microvolts. In the case under discussion, with the left hand instrument set to 999.9 mV, a null was produced with the right hand instrument set to 18.70 mV . So the Weston Cadmium cell EMF is calculated to be 1.0186volts.