Repair and Restoration of an Early

Leeds & Northrup Resistance Bridge

I bought this bridge at the Rochester, NY hamfest on June 1, 1996 for $40 or $45, I can't remember exactly. It was extremely dirty, but didn't have any major cracks or missing pieces. It appealed to me because of its large exposed brass switch contacts, and plug selected range resistors.

Though I do not have any L&N catalogs, their measurement equipment is featured prominently in many early engineering texts. In Dawes' 1937 Electrical Engineering text (Vol. 1, Direct Currents, 3rd edition, pg. 168) this exact model bridge is shown. A similar model with single (not dual) range resistors is shown in texts from 1910, and this exact model in a metal case appears in a text from 1950. Originally I thought the bridge might have been built around 1940, but subsequent communications with retired bridge experts from L&N and GR Corps. placed the date closer to 1915*.

The internal resistors are typical for the time period, being of tubular construction, and coated with paraffin. Laws' 1917 Electrical Measurements text gives the following description of their construction:

Resistance Coils.— Formerly it was the universal practice to wind resistance coils on wooden bobbins, but, in the better class of work, these bobbins have been replaced by metal spools (see Fig. 57). A layer of shellaced silk which is dried out by baking before the coil is wound serves to thoroughly insulate the wire from the metal spool.

Non-inductive windings are always employed. The wire is arranged in a bight before it is wound upon the bobbin and the two wires are kept side by side in the coil.

If possible, the winding is concentrated in a single layer, for as all the heat must be dissipated through the surfaces of the coil, one wound several layers deep with a large wire is not superior to one wound with but a single layer of small wire.

After it is wound, the coil is shellaced and then baked for 10 hours or more at 140° C.; this frees the entire coil of moisture and alcohol and at the same time anneals the wire. After baking the coil should be given a protective coating of paraffin.

The resistance wires are hard-soldered to copper terminals which in turn are soft-soldered to the working terminals.

The prime requisite of the resistance material used for winding coils is permanence. In addition, its temperature coefficient should be small, its thermal e.m.f. when opposed to copper, low, and to obtain compactness, its resistivity should be high. Alloys rather than the pure metals are used for they have smaller temperature coefficients and higher resistivities. To settle the all-important question of permanence prolonged investigation is of course necessary. Up to the present time, the alloy which has most commended itself is that known as manganin. Other alloys are used for certain kinds of work but manganin has been under critical examination longer than the others and its properties are more definitely known.

Edward Weston discovered in 1889, that alloys of copper and nickel containing some manganese have very small temperature coefficients and high resistivities. Investigation has shown that the particular alloy known as manganin is, when properly employed, sufficiently permanent for resistance coils and resistance standards.

After a cursory inspection and dusting off, I decided to test the bridge. Even though it was intended as a display piece, I was curious how well it worked. The answer was– not very! The battery and galvanometer switches were intermittent, and the 1KΩ decade appeared to go open above 3KΩ. Measuring all the internal resistors showed that three of the 1KΩ units were bad, as was one of the 10KΩ range selecting resistors.

A clue was provided by the missing paraffin on one of the 1KΩ resistors. It had obviously been overloaded. Each of the failed resistors had a broken wire next to the working terminal. Most likely a severe overload caused the exposed wire to overheat and fail. The insulated wire had better short term heatsinking, and survived.

As the break was very short, I elected to simply reconnect the resistance element with about three millimeters of small copper wire. The repair could have been made with resistance wire, and the values retrimmed, but this didn't seem worth the trouble.

After repairing the resistors, the bridge worked fine, however it was not quite within tolerance. Careful measurement of the resistance values showed all the decades to be nearly perfect, except the 1KΩ decade. These values were slightly low, even the ones that had not been repaired. Apparently the overload had done some permanent damage. Though it would be possible to bring all the resistors into tolerance by adding a small piece of resistance wire, the risk of further damage wasn't worth it. I elected to leave well enough alone. Note, however, that these bridges are typically better than 0.05% accurate if treated well.

Here are the measured resistor values for each decade:

.1 1 10 100 1000
0 .040 .040 .040 .040 .040
1 .140 1.040 10.055 100.21 1000
2 .240 2.041 20.06 200.2 1999
3 .340 3.042 30.07 300.3 2998*
4 .440 4.042 40.09 400.3 3996
5 .541 5.043 50.10 500.3 4994*
6 .641 6.043 60.12 600.3 5992*
7 .741 7.043 70.13 700.4 6991
8 .841 8.043 80.16 800.4 7988
9 .941 9.043 90.18 900.3 8986
10 1.039

Note that the residual test lead reading of .040Ω should be subtracted from each reading. The * denotes resistors that were repaired.

The measured values of the range setting resistors:

9886 9976 1000.0 999.7 100.1
1.009 1.009 10.014 10.020 100.1

The final step was disassembling all the switches, cleaning the contacts and leaves, then reassembling the instrument. The lettering on the top plate was re-filled with white enamel where necessary, and the sheet metal and wood based cleaned and reattached. Early electrical measurement books suggest white petroleum jelly to lubricate the switch contacts, so a thin film was applied.

In general, my philosophy is not to "over restore". The instrument should look the way it probably looked shortly after being put in service. Well cared for, but ready to do useful work.

bridge image

*Special thanks to Henry Hall of GR for contacting Wes Shirk of L&N, who identified the bridge as a model 4725, and confirmed Henry's date of around 1915. The 4725 was one of the earliest bridges built by Morris E. Leeds.

10 June 1996, appended date information 13 September 2000,

Conrad Hoffman