© Brooke Clarke 2014

ZM-14A/PSM-2 Megohm Megger meter
ZM-14A/PSM-2 Megohm Megger meter



When testing high resistance values and using a very sensitive meter the test voltage needs to be high in order to get to 1,000 Meg Ohms (1 Gig Ohm) so the Meg Ohm Meter, Megger for short, was developed.  The test voltage is regulated at about 500 Volts and the max resistance is 1,000 Meg Ohms so this is a 2 M Ohm per Volt meter.


I got a very good deal on this ZM-14A/PSM-2 because one of the high voltage terminals has been broken and when someone tried to repair it they broke 
Navy Stock No. G17-M-30366-2334
Ser. No. LC 3119 73  Cont. No. NObs-R-57505
Boston, Mass. U.S.A.
Holtzer-Cabot manufactured many products that depended on electromagnetism such as AC and DC motors (like the one used in the Western Union 5A Stock Ticker) as well as AC and Dc generators both with shaft drive and hand crank types like used in very early wooden magneto wall phones.  This Megger contains a generator with a horseshoe magnet and that magnet is cylindrical and about 3/4" diameter, so this generator is different from those used in the TA-312 military field phone.
Photo taken after adding comments to the patent drawing.
Note red horizontal resistor 5.0 MEG just as was computed.

Rectifier is marked:  U 30RP (maybe wrong)  Let me know
                  Megohm Megger meter
To the left of the magneto you can see part of the round horseshoe
                  Megohm Megger meter

More on how it works is in the patent below.


2623098 Measuring instrument, Johnson Ernest W, Vernon Durbin (National Pneumatic Co.) Dec 23, 1952 - 324/722 ; 322/46; 322/96; 340/642
Neon lamps are used to regulate the output DC voltage, unlike the Biddle that uses a mechanical governor on the magneto.
The light from the Neon lamps is visible at the top so that the operator knows to crank fast enough to keep them lit.
Voltage from A to B (across filter cap (22) is about 500 Volts DC.
8: magneto
12: hand crank
19: HV rectifier
21: no longer used bias resistor for vacuum rectifier
22: filter cap (points A and B are across this cap)
28: Weston 306 90 uA meter movement
        The calibration resistor (34) may be to bring the parallel combination to 100 uA full scale).
30: short current current limiting resistor
      If the calibrated meter/R34 combination is 100 uA fs then this resistor would be 5 M Ohms.
      Then half scale would be 25 M Ohms.
32: String of Neon lamps
34: Meter calibration shunt
36: Test Terminals
42 plastic rod light pipe with ends (44) rounded and frosted
47: guard resistor for leakage paths between terminals (36) and case
            Megger MegOhmmeter patent 2623098
  858335 Apparatus for measuring electrical resistances,Sydney Evershed ( Evershed Vignoles Ltd, Sydney Evershed), Jun 25, 1907, 324/706 - contains geared up AC generator
1681367 Electrical testing instrument, Rolfe George Berkeley (Evershed Vignoles Ltd), Aug 21, 1928, 324/722 -
                Biddle megger - no voltage regulation instead uses a governor to regulate the magneto speed
1918834 Voltage indicating apparatus, Crago Paul H (Union Switch & Signal Co), Jul 18, 1933 - 324/122, 340/662, 324/72.5 - not a megger
2260234 Instrument for measurement of electrical resistance, Berkeley George (Evershed Vignoles Ltd), Oct 21, 1941, 324/704, 310/156.47 -
                sync mechanical rectifier
2272239 Device for measuring electrical resistance, William J Delmhorst (none), Feb 10, 1942, 324/714 - battery & vacuum tube
2326313 Alarm circuit, Trucksess David E (Bell Telephone Labs), Aug 10, 1943, 340/645, 327/545, 361/187 - failure of rectifier trips alarm
2429764 Electric fence indicator, Sidney A Moore (Prime Mfg Co), Oct 28, 1947,
                340/635, 307/150, 324/122, 315/189, 324/72, 256/10, 307/132.00R
                Uses Neon lamps to regulate line voltage
2460095 Electric fence indicator, Sidney A Moore (Prime Mfg Co), Oct 28, 1947,
                340/635, 307/150, 324/122, 315/189, 324/72, 256/10, 307/132.00R


Digital Multi Meters
Test Equipment
Microwave Test Equipment


Brooke's: PRC68, alphanumeric web page index, contact, Products for sale