A Span of (Wheatstone) Bridges
Thomas B. Greenslade, Jr.
Kenyon College, Gambier, Ohio 43022
Wheatstone electrical bridges, in many configurations and sizes, have been a staple of my scientific life. Indeed, my doctoral research was based on the use of a very good Wheatstone bridge. Regretfully, the bridge circuit has passed out of the undergraduate physics curriculum in recent years. The history of the bridge can be found elsewhere. (Ref. 1 and 2) My aim now is to illustrate some of the many varieties of Wheatstone bridges that I have found in my travels. With the exceptions of the apparatus in Figures 5 and 10, all of the instruments are in the Greenslade Collection.
The traditional form of the bridge is shown in Fig. 1, drawn from Wheatstone’s 1843 Bakerian Lecture that is a primer on how to make electrical measurements. (Ref. 3) The four resistances are connected to form a diamond, and the source of EMF and the detector are connected across the diagonals. Wheatstone used the same diamond shape in the telegraph instruments that he developed with William Cooke starting about 1840.
The face of the student bridge in Fig. 2, sold by A.P. Gage of Boston ca. 1890, shows this diamond configuration. In some student bridges the diamond shape is morphed into a triangle.
The little bridge in Fig. 3 is listed in the 1912 Stoelting of Chicago catalogue at $5.00. The two upper arms can be 1, 10 or 100 ohms, and the unknown and the calibrated variable resistors are connected at the points marked “X” and “R”.
The long and heavy Leeds & Northrup bridge in Fig. 4 is a Reversible Meter Bridge, and is listed in the 1907 catalogue at $115. This must have not proved successful, for it does not appear in subsequent catalogues. The scale is engine divided, and there are both coarse and fine adjustments to the position of the slider on the slide wire. I keep this instrument outside, as the reversing switch in the middle uses mercury cups.
The portable Wheatstone bridge in Fig. 5, with its circular shape, is perhaps unique. The device was made by the firm of J.H. Bunnell, which was founded by the Civil War telegrapher, Jesse Bunnell, and made a wide range of electrical apparatus in addition to telegraph instruments. The galvanometer is on the right-hand side. The apparatus is in the collection of David Wahl, who also took the picture.
The Leeds & Northrup Type S Portable Test Set in Fig. 6 was popular for testing telephone and telegraph lines. This cost about $90 when it was made in 1917, and full descriptions for the use of the various tests are printed on the card inside the upper lid. The galvanometer was a stock item and could be bought separately for $15.
Other manufacturers also made portable test sets. The large instrument in Fig. 7 was made by the Thompson-Levering Co. of Philadelphia. This firm was founded in 1879 and was merged with General Electric in 1892. Again, the galvanometer was available as a stand-alone instrument. In 1950 a previous owner traced out the circuit and tacked it inside the lid. This instrument, with its shorting pegs, is a good deal harder to use than the next-generation of test sets that used switches to choose the resistances; see Fig. 6, for example.
The portable Wheatstone bridge in Fig. 8 has brass rings on either side used to lift it from its protective box. The suspended-needle galvanometer with an iron compensating bar seems like an anachronism. The instrument was made by E.S. Greeley & Co. of New York, “Manufacturers of Standard Electrical Test Instruments.” This company, in business from 1886 to 1897, was unusual in starting as a maker of telegraph instruments and supplies.
Some bridges are large, heavy and definitely non-portable. The Wheatstone bridge by Otto Wolff of Berliin Fig. 9 was once used in the testing laboratory of Macy’s department store in New York City. The amount of brass in its switches is considerable, for it weighs 25 pounds. The coils are made of Manganin, and it is stated to be “richtig bei 20ºC”.
The enormous Wheatstone bridge in Fig. 10 is one of a dozen pieces of apparatus bought by Denison University from Leeds & Northup in 1905 to replace apparatus lost in a fire. In the 1907 L&N catalogue it is listed as a “Decade Set and Wheatstone Bridge”, and cost $275. I found that this bridge was at the limit of my strength to lift, indicative of the amount of brass in it. It cost $275.
Some are specifically for student use. Fig. 11 below is a small bridge that once found in multiple copies in almost all undergraduate physics departments in the United States. This one dates from 1919 and cost $55.
The 1927 Leeds & Northrup catalogue of “Apparatus for Electrical Resistance Measurements” lists the bridge in Fig. 12 at $175. This has holes in the metal sides to the base to allow air to circulate and cool the coils when used under high energy dissipation conditions.
The Leeds & Northup bridge in Fig. 13 is serial number 9089, which dates it as being made in 1907. This is a classic Post Office Box design; the reference is the British Post Office Department that controlled the telegraph system. The instrument and its parts were bread and butter items for Leeds & Northrup: the 1907 “Electrical Measuring Instruments” catalogue notes that this was made from stock parts: “standard blocks, plugs and resistance spools, which we make up in large quantities for our portable testing sets…” It cost only $40.
Some bridges are multipurpose. When I was an undergraduate in the 1950s, the General Radio Type 650A Impedance Bridge (right-hand side of Fig. 14) was the work horse of the electrical measurements laboratory. This was patented in 1928-1929 and can be used as a standard Wheatstone bridge. However, when the power supply is a 1000 Hz alternating current signal and the detector a pair of headphones, it can be used as to measure the impedance of a coil or the capacitance of a condenser. In 1935 it cost $175, and was so large and heavy the GR code name for it was “beast.”
The General Radio Type 635A bridge on the left-hand side of Fig. 14 is the skeleton of a bridge. It contains only the hummer for AC measurements and a pair of 4.5 V batteries for DC work, plus a 10,000 ohm rheostat with a logarithmic scale. The open drawer contains a series of fixed capacitors of resistors so that a custom bridge may be constructed for a particular measurement.
One problem with the original form of Wheatstone’s bridge is its inability to measure very low resistances. The resistance of the wires connecting the bridge to the sample can be comparable to that of the sample, leading to errors. Lord Kelvin (1824-1907) devised a modification that allowed the resistance of a few tens of centimeters of thick copper rod to be measured. Fig. 15 shows a Leeds & Northrup Kelvin Bridge from 1920 that sold for $70. These survive in a few college and university physics collections, but the apparatus in Fig. 16 is quite rare. The two parts of the bridge are the Adjustable Standard Low Resistance ($300) and the Box of Ten Ratio Coils ($100). Resistances ranging from 1 ohm down to 1×10-8 ohm can be measured with this apparatus.
Finally, the Wheatstone bridge is also a part of a delightful visual pun. Fig. 17 shows the logo of the Cambridge Instrument Company: a cam surrounded by a bridge.
1. Thomas B. Greenslade, Jr., “Wheatstone’s Bridge”, Phys. Teach., 43, 18-20, (2005)
2. Thomas B. Greenslade, Jr., “Bridges I Have Crossed”, IEEE Instrumentation and Measurement Magazine, 12, 16-20 (2009)
3. It is well known that the first user of the bridge circuit was Samuel Hunter Christie in 1833. Wheatstone acknowledges this in his Bakerian Lecture. For many years it has been assumed that Christie was the son of William Christie of the eponymous auction house, but recently it has become clear that he was the son of a different William Christie. I rather liked the legend better!