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by Bob Mills

First, a Little about Resistance.

Expressed in absolute units, the ohm is equal to 109 electromagnetic units of resistance. This would be inconveniently small for practical purposes so it was decided by the International Congress of Electricians, held in Chicago in 1893, to adopt the International Ohm as the standard unit of resistance. This standard is based upon the resistance offered by a column of mercury, 14.4521 grammes in weight and a length of 106.3 centimetres at the temperature of melting ice.

Man has had a number of attempts at defining the exact measure of the ohm. One of the first was in 1862 and was recommended by the British Association Standards Committee. This measure was known as the British Association Ohm or B.A. unit and consisted of a coil revolving in the earth's field and acting as its own galvanometer. This did not prove very satisfactory and rather inaccurate at 0.9866 of what was to become the international ohm.

Later in 1884, an attempt was made to correct the error and the B.A. Committee issued the Legal Ohm (which was never legalised in England). This proved to be 0.9972 of the international ohm. Then there was the Siemens' ohm (probably from around Germany) which measured the resistence of mercury 100cm long and 1 sq. mm in cross section at 0 degrees C. the table at right gives relations.



International Ohm



Legal Ohm



B.A. Ohm



Siemens' Ohm


Resistance Wire.

For practical use in measuring resistance in the field, sufficient accuracy can be obtained by using resistance wire usually wound on a spool. Wire used in standard resistance must have the following qualities:

1. High specific resistance.
2. Small change in resistance with varying temperature.
3. Invariability of resistance with time.
4. Should not be readily acted upon by moisture or chemicles.
5. Small thermal EMF.

Pure metals fail in most of these requirements however alloys offer the best adherence to these essentials. Around the start of the 20th century the following alloys were often used for standard resistances: German Silver, Platinum Silver, Manganin, Platinoid and Constantan.

Of these, German Silver (60% copper - 25% zinc - 15% nickel) was extensively used in the 1800's but came to be condemned as it failed condition (3) above, varying with time particularly in moist climates or if subjected to much temperature variation. This was thought to be mechanical deteriation of the wire due to crystallation of the zinc. Thus, all alloys containing zinc, were distrusted for use in resistances from about 1900 onwards.

Platinoid is a well known and widely used alloy, having similar characteristice to German Silver although its composition is entirely different (platinum + silver + 1 to 2% tungsten). Its greatest asset is that it stands the test of time in a most satisfactory manner.

The alloy with the trademark name of Manganin (84% copper - 12% manganese - 4% nickel) came on the scene in the mid 1890's and performed well in most areas although it was subject to some action from moisture or chemicles. To minimise these effects, wires were well covered with shellac, baked on, before assembly onto spools. Manganin was popular in good quality English made resiatance boxes from the early 1900's and the name is usually stamped on the top of the box alond with the temperature at which the resistances were calibrated (usually 15.5oC).

Resistance Coils.

The resistance coils that go to make up a resistance box of 75 or more years ago consist of wire of one of the abovementioned alloys, covered with silk or paraffined cotton, wound with great care and adjusted in length to have resistance of a definite number of ohms. To avoid self induction they are wound in a non-inductive manner (doubled back on themselves) as shown in figure 1.

Each end of a coils is soldered to a brass piece, as in figure 1, the first coil is soldered to brass A and B whilst the second is soldered to brass B and C. The brass pieces are themselves fixed to a block of ebonite which forms the top of the resistance box. Sufficient room is left between each adjacent block to allow brass plugs to be inserted. While the plugs are in position current flows through the plug which shorts out the resistance. On taking the plug out, the current is forced through the resistance. By this means the amount of resistance thrown into the circuit can be controlled. A typical example is shown in figure 2.

The series of resistance coils are chosen to allow any desired value of resistance to be inserted in a circuit. A typical box may contain coils with the following numbers of ohms: 1, 2, 2, 5, 10, 20, 20, 50, 100, 200, 200, 500, up to 10,000 in some boxes. Values less than 1 ohm are often included and other combinations will often be found. The above combination allows any value of resistance to be selected, for example, 456 ohms would be made up of 200 + 200 + 50 + 5 + 1 by taking out just 5 plugs.

Using the Resistance Box.

A simple use of the resistance box is to wire it to a battery via a galvanometer and two way key to measure an unknown resistance as in figure 3. Read the galvanometer deflection with the unknown resistance in circuit then change over and adjust the resistance box until the same deflection is obtained. Read off the resistance in circuit and there you have it. Other simple uses include adjusting relay timing, balancing duplex or quad telephony and in laboratory work.

The Wheatstone Bridge.

One of the principal uses of the resistance box is in the Wheatstone Bridge. The so called "Wheatstone" bridge was invented by Christie, and improperly credited to Charles Wheatstone, who simply applied Christie's invention to the measurement of resistances.

The principle of the Wheatstone Bridge is shown in figure 4. The purpose is to adjust the reostat (resistance) in the B arm so there is no current flow through the galvanometer. Thus, in a simple case, if the resistance of arm A equals arm C and the resistance in the B arm has been adjusted so that the deflection on the galvanometer is zero, then the unknown resistance equals the value on the reostat or B arm of the bridge.

Many resistances boxes were made almost exclusively for use in Wheatstone Bridges and resistances were often laid out in 3 groups forming the A, B and C arms of the bridge. A typical bridge connection is shown in figure 5. Twi "tapping" keys were needed to connect the testing battery to the bridge and to connect the galvanometer into circuit. These keys were also often found built into many resistance boxes. In some cases, even the galvanometer was a part of the box. A box complete with tapping keys is shown in figure 6.

When using the keys, it is important to operate the battery key before operating the galvanometer key to avoid damage to the movement due to the sudden swing of the galvanometer needle which occurs due to self induction.

The most usual use of the Wheatstone Bridge in day to day telephony and telegraphy was the location of cable faults. By determining exactly the resistance of a fault and comparing it with a pre-determined map, it was possible to send repairmen out to the exact location of a fault, accurate to within a few feet.

Types of Boxes.

The majority of resistance boxes, whether simple resistance boxes or more complicated bridge boxes, used brass plugs to throw resistance in and out of circuit. Resistances were usually laid out in rows with the value of resistance stamped into the brass or the ebonite base. Some sophisticated boxes arranged the coils in circles of tens or units as in figure 7. Another type of box used Bakelite cobered "caps" (as shown in figure 8) in an attempt to provide better contact between brass and plug. More modern boxes, some made in the last 20 or 30 years, used rotary switches to select the value of resistance required.


There were a number of "specialist" manufacturers of resistance boxes. Perhaps the most famous was Muirhead & Co. Ltd. of Westminister, England. One Muirhead bridge box in the posession of the author was made in 1911, came with a satin lined hinged lid, and had all connection details engraved in the brass blocks including "carbon", "zinc", "galvanometer", "line", etc.

Other manufacturers include Silverton, London, James White, Glasgow, Gambrell Bros. Ltd., London, Leeds and Northrup, Philadelphia, Knott, Boston, J.H. Brunnel, (USA) and many, many more. Simple boxes have been seen made in "Singapore Workshops" (part of Singapore P&T it is assumed). Then there are those unmarked which may have been one off jobs for use in factories or short runs made by individual telephone companies.

In old telephone exchanges charged with maintaining long distance carrier circuits, the resistance could be a very fancy affair, built into the test panel and featuring rotary switches for resistance adjustment, plug and cord connection to the line under test and an extremely sensitive mirror galvanometer to enable fault locations to be performed quickly and accurately.


A resistance box may be of little practical use to todays telephone collector. However they are, in good condition, a most attractive part of any collection particularly when the timber is in good condition and the brass is polished and covered with gold coloured lacquer. They provide a link with a little known part of telephony and telegraphy history, that of testing and fault location.


The Electrical Engineer Institute, Book 14, "Laboratory Instruments and Measurements", 1904.
Modern Electric Practice by Magnus Maclean, Editor, Vol 1, 1906.
Audels New Electric Library, Vol. III, 1943.

Originally printed in January 1996 edition of the ATCS Newsletter.

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