"The HP 412A is a multipurpose meter designed to measure dc voltages, current, and resistance with laboratory accuracy and yet be of great utility in production-line test-bench work.
The extreme stability of the 412A makes it easier to use by eliminating the need for constantly re-zeroing the meter. The stability of the 412A is such that the manual front-panel, zero-set control has been omitted" (hp, Catalog, 1968)
BRIEF SPECS
| Voltage | 1mV to 1000V |
| Current | 1 mA to 1 A |
| Resistance | 1 ohm to 100 Mohm |
| Output | 1V f.s. |
| Power | 25 watts |
| Dimensions | 11-1/2" x 7-1/2" x 10" |
| Weight | 12 lbs |
| Technology | tube |
| Price (for 1968) | $ 450 |
The 412A DC V-A-Ohm-meter is very stable due to it's opto-chopper based design (like the later 410C). High accuracy up to 1% (!) is obtained from feedback design. The chopper consists of a little ac motor, a blade and lamp-photoresistor elements. Result of powering the motor from ac main is extreme high ac rejection. This 412A is one of the few meters which can measure the DC of a combined dc/ac signal. It's built around 1970.
