Hartmann & Braun Insulation Tester

(Voigt & Haeffner or Union Electric )

Serial number 450941

This "Union Electric" insulation tester was in a very grubby condition when I purchased it on Ebay for 10. The terminals were incomplete and two of the internal resistors were open circuit. The generator was in excellent condition and as can be seen from the picture was well made and housed in a well designed and constructed mahogany box with dovetail joints, the lid has a bevelled edge on the hinge side so that it can be folded flat against the rear of the box. The terminals and the black knob to the right of them are replacements.To the left of the meter is a small compartment to stow the generator handle when not in use.

Unlike other insulation testers that I have, this one is intended to be operated whilst hanging from the operator's neck, the right hand turning the generator handle and the left depressing the button to unclamp the meter needle. I soon discovered that the brass shaft for this knob is energised when the handle is turned!

This is a close up view of the meter and the unusual mushroom shaped pivoting mechanism surrounding the meter movement. At the left hand side of the mechanism can be seen a small horizontal and a similar vertical metal plunger. The former is operated by the push button knob adjacent to the output terminals and serves to unclamp the meter needle. The other is operated by an internal lever which serves the same purpose but will permanently lift the clamp and render the push button knob ineffective but as can be seen from the instructions (see below) this lever should automatically operate once the correct reading has been obtained and clamp the pointer. This should occur when the speed governor fly weights swing outwards.

Here are two views of the generator showing the steel gearing, the commutator and the centrifugal governor. Note how the unit is completely self contained and equipped with contacts which mate with those on the remainder of the unit shown on the picture below.

The series resistor is made up of three banks of three wire wound wooden spools. Two of the banks are labelled 335,000w. As two of these banks were open circuit I have left these in place but shunted them with three high stability resistors totalling 335 000 ohms. The lever for unclamping the meter can be seen to the right of the resistor spools.

The meter has a full scale deflection of 1.5mA and a resistance of 215 ohms.


The following have been transcribed from a poor a very poor contion instrument owned by Christopher King:

In order to obtain accurate results the following directions should be carefully observed:
The circuits to be tested must not be connected to any other source of pressure.
One terminal of the instrument is to be connected to the circuit under test, the other terminal to earth.

Commence by turning the magneto slowly and steadily (about 50 revolutions of the crank per minute), this speed should be continued for 30 seconds, afterwards gradually increase speed until the small trigger under the automatic stop flies out and releases it, thus locking the pointer. The indication is direct in megohms.

After each test, press the button. on the front side of the case to reset pointer.
The screw on the back of instrument serves for correcting the zero position of pointer.
The inductor must he greased periodically with’ some drops: of good oil, and the collector with a drop. of petroleum, For this purpose: remove the 8 screws from the wall of the apparatus. When the board has been taken off the oil holes become visible.


The Union Electric Company Ltd. was until the outbreak of World War One a subsidiary of Voigt and Haeffner which had a factory in Frankfurt, where they made electrical apparatus they also had works in Birmingham. The name then changed to " The Switchgear Construction Company, Ltd." which incorporated both the British based firms. The majority shareholders were still German. The Union Electric Company installed electric lighting in the public galleries of the Natural History Museum in London as well as lighting for London theatres.

I guessed that this instrument was made in Germany prior to 1914, the trade mark device on the card either side of the meter scale is H&B which indicates that it was made by Hartmann & Braun, also of Frankfurt.

The Institute for Urban History in Frankfurt a.M. is in possession of the Hartmann & Braun company archive, including a number of books listing the scale numbers of their instruments from 1889 until 1959. The database which can be accessed here: https://www.alte-messtechnik.de/hub/skalennummer.php confirms my initial belief by indicating that it was most probably made in October 1913

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