56 September/October 2018
1.0193 +.0002 volts to be exact.
Calibration labs in companies throughout the world
used these cells to calibrate their voltmeters so the
specifications of the electrical components and equipment
they produced would be comparable for all users. The
cells could only supply a few microamps (never more than
100 µa) when used. Typically, a laboratory potentiometer
which used zero current from the cell when balanced, was
used to generate other precise higher voltages to calibrate
There was an interesting caution in the cell’s operating
instructions, “If a cell is shorted for 30 minutes, allow five
weeks for it to recover to within 75 µV.” Lesson: Don’t
short them or it’s a long wait.
TRACEABILITY TO NATIONAL
BUREAU OF STANDARDS (NBS)
Although Weston cells were very stable, they still
needed to be periodically checked against the nation’s
super-accurate primary standard cells located at the
National Bureau of Standards (NBS) in Washington, DC.
Calibration labs and cell manufacturers would
periodically send their cells to Washington and
NBS would issue a certificate stating their precise
measured voltages, good to one microvolt. The most
stable type of cells — called “saturated” cells — were
so delicate that they could not be tipped more than
45 degrees and had to be hand-carried all the way to
and from NBS.
Cell manufacturers (like Eppley) would also
maintain super-accurate saturated reference
standards at their facilities, so the individual
secondary cells they sold would be traceable back to
NBS. Figure 7 shows the slightly wrinkled certificate
that came with my Serial Number 864673 Eppley
Standard Cell, certified to six digits. My secondary
cell is the “unsaturated” type which is not sensitive
to tipping and can be shipped in the regular mail.
NOTHING IS EVER EASY
Just owning the neat cell was not enough. Now, I
wanted a “good” digital voltmeter to measure it, not my
four-digit Fluke. I wanted to know exactly what the voltage
was, to at least six digits or more. So, I checked the price
of a refurbished 7-1/2 digit HP: $2,550. Gulp! What about
eBay? How about a vintage 6-1/2 digit HP3456A for $99?
Bingo! Check it out in Figure 8.
When it arrived, all the digits came on. However,
pushing the Test button triggered an error of “- 4.0000”
which made me suspicious. So, I shorted the input
terminals and it displayed all kinds of random digits, not the
0.000000 I expected.
After several days of taking it apart, measuring different
voltages, and searching for people with the same problem,
I found the answer. Not good! The older HP3456A
voltmeters (like mine) had a design flaw. The three ROMs
on Board #A4 tended to lose their memories after several
years. I said a few words I can’t repeat.
After more research, I found some enterprising
experimenters who had figured out how to replace the
bad ROMs with more modern EPROMs,
like 2716s or 2732s. It sounded like a fun
challenge until I realized how many hours
it would take to pull the chips, modify
the address pins, download the files, and
burn the new EPROMs. Even then it might
not work and it would still need to be
So … I caved in and bought another
HP3456A from a regular test equipment
house, for three times the money, and they
calibrated it to the original specs. When
it arrived, I checked to see if the ROMs
FIGURE 7. The manufacturer provided a calibration certificate along with
each cell, good for one year.
FIGURE 8. After 39 years, the voltage produced by
this cell had only dropped 0.115 millivolts.