Electric Currents and Meters
Edison BSCO Primary Battery #10422
Thomas Edison, Inc., Orange, NJ
The porcelain jar has one electrode between a pair of other electrodes. They are all too corroded to tell what materials they are made of. The last patent date on the cover of the jar is 1911.
Microphone Hummer #10544
Leeds & Northrup, Philadelphia
A 1000 Hz tuning fork is driven electrically by a coil-interrupter combination. A second coil on one of the tines of the fork is a pickup coil to provide an electrical output signal.
Sir William Thomson’s Patent Electric Balance #10001
J. White, Glasgow
This form of current balance was used as a secondary standard of electric current. Thomson’s earliest form dates from 1882 and this more accurate one from 1887. This particular instrument was purchased by DeWitt Bristol Brace in 1889, so it was the "state of the art" in electrical measurements at that time.
Parallel electric currents attract or repel each other, depending on whether the currents are in the same or opposite directions. In this instrument there are two pairs of fixed coils and between each of the pairs is a moveable coil. The fixed coils are connected in opposition so that one attracts and the other repels the moving coil to increase the sensitivity. Current sent through the coils causes the balance beam to tilt. The balance is restored by placing one of the calibrated weights in the V-shaped trough on the right and by moving the slider along the graduated scale. Strings are provided to move the slider even when the protective glass cover is in place. Since the position of the slider is proportional to the square of the current, the scale is marked off quadratically. Different current ranges are available by using different weights. Five models were available which covered the range from 0.01 to 2500 amperes. Alternating currents can be read as well as direct currents.
The instrument is adapted and calibrated for use in measuring potential (in volts) or power (in watts) depending on the setting of the lever in front under the base. A large shunt is connected on the back to allow for measuring large currents. We have the two original calibration charts signed by William Thomson himself.
Thomson current balances were widely used for calibrating other instruments but by World War I they were superseded by more convenient systems involving springs rather than by utilizing gravity.
References: George Green and John T. Lloyd, Kelvin’s Instruments and the Kelvin Museum, Glasgow, 1970, pp.28-29; Frank A. Laws, Electrical Measurements, New York, 1938, pp.77-78; Whipple Museum Catalogue 8: Electrical and Magnetic Instruments, 1991, No. 156.
Leeds & Northrup, Philadelphia
The system of a rotating coil in a field of a permanent magnet was used by William Thomson in 1870 but was generally neglected until Marcel Deprez (1843-1918) and Jacques d’Arsonval (1851-1940) reintroduced it in 1882 to be used in galvanometers. This arrangement has been used in most voltmeters and ammeters ever since. The coil is suspended either on a torsion fiber as here, or by jeweled bearings.
References: John T. Stock and Denys Vaughn, The Development of Instruments to Measure Current, Science Museum, 1983, chapter II; Robert Bud and Deborah Jean Warner, Instruments of Science: An Historical Encyclopedia, New York, 1998, pp.257-59.
Nalder Bros., London
This is a British example of the suspended coil and permanent magnet galvanometer.
This glass-topped meter in a wooden case is of the classic d’Arsonval type with a permanent magnet and a pivoted, rotating coil.
Siemens Bros., London
In an electrodynamometer the magnetic field is provided by a current carrying coil instead of a permanent magnet. It was invented in 1845 by Wilhelm Weber (1804-1891), Professor of Physics at Leipzig and Göttingen. This is operated in the null mode, i.e., the fiber suspending the rotating coil is turned so as to bring the coil back to its rest position, the current then being read from the angle of rotation of the fiber. This was a secondary standard for current measurement until the 1920s when it was replaced by the more convenient direct reading meter patented by Edward Weston.
References: John T. Stock and Denys Vaughn, The Development of Instruments to Measure Current, Science Museum, 1983, p.39-40; Gustav Wiedemann, "Die Lehre von der Electricitat, 1895, pp. 64-66; Whipple Museum Catalogue 8: Electrical and Magnetic Instruments, 1991, No. 151.
This is similar to the preceding electrodynamometer (#10135) with a single-turn coil rotating in the field provided by another coil.
Reflecting Astatic Galvanometer #10093
Elliott Bros., London
The term "astatic" refers to an arrangement by which the disturbing effect of the Earth’s magnetic field is eliminated or reduced. This is accomplished by the use of a curved neutralizing magnet above the instrument, or, as here, by using pairs of opposing coils on either side of the magnetic needles. This is a form introduced by William Thomson in 1863. Its sensitivity of about 10-11 amperes pushed the sensitivity to its limit for the time.
References: James W. Queen & Co. Catalogue I-66 Electrical Testing Apparatus, 1887, p.195; John T. Stock and Denys Vaughn, The Development of Instruments to Measure Current, Science Museum, 1983, p.24; Max Kohl Catalogue No. 100, p.894; Emilio Segrè, From Falling Bodies to Radio Waves, New York, 1984.
School Galvanometer #10101
Max Kohl, Chemnitz
This glass-domed galvanometer has a pair of suspended, lightweight magnets, one of which is inside a coil. It can be used as a galvanometer, a tangent galvanometer, a compass, a differential galvanometer, and an astatic galvanometer. It is not calibrated, but is stated to have a sensitivity of 40 microamperes per division.
Reference: Max Kohl Catalogue No. 100 (c.1927) p.891.
Edison General Electric Co., New York
This is an example of the moving-iron type meter in which a curved rod of iron is drawn into a solenoid carrying the current to be measured. Since the deflections are not proportional to the current, the scale had to be calibrated or a calibration table had to be provided. In the 1880s meters of this type were used by electric plants for rapid measurements of large currents. This meter has a full-scale reading of 132 amperes. On its glass cover it is labeled "Edison System Aperemeter."
Reference: John T. Stock and Denys Vaughn, The Development of Instruments to Measure Current, Science Museum, 1983, pp.34-36
Voltmeter Multiplier #10284
Weston Electric Instrument Co., Newark, NJ
Typical voltmeters consist of a galvanometer, which measures small currents, and a set of large series resistors that can be switched to provide different scales. The resistors (or "multipliers") are usually contained in the voltmeter case, but for high voltages, external multipliers, calibrated for the particular instrument, are used. This multiplier is designed for use with the Weston Direct Reading Voltmeter.
Reference: Walter C. Michels, Advanced Electrical Measurements, Toronto, 1941, p. 52.
Ammeter Shunt #10250
Elliott Bros., London
While a voltmeter needs a large series resistor to read high voltages, an ammeter requires a small parallel resistor (or "shunt") to handle large currents. This shunt has plugs reading "1/9, 1/99, and 1/999" to indicate the fraction of the current in the meter itself, the rest being in the shunt.
References: John T. Stock and Denys Vaughn, The Development of Instruments to Measure Current, Science Museum, 1983, p.21; Whipple Museum Catalogue 8: Electrical and Magnetic Instruments, 1991, No. 305.
AC and DC Ammeter #10143
Westinghouse Electric Co., Pittsburgh, PA
This moving-iron meter was made by George Westinghouse, Thomas Edison’s rival in the electrical business. Westinghouse was the champion of the use of alternating current while Edison preferred direct current. The moving-iron meters could be used equally well with either AC or DC and this meter is marked with both designations.
Wheatstone Bridge #10104
Otto Wolff, Berlin
This circuit, studied by generations of physics students, was actually invented by S. Hunter Christie in 1833. When Sir Charles Wheatstone called attention to it in 1843, he gave full credit to Christie, but somehow Wheatstone’s name became inextricably associated with it. The bridge is used to measure unknown resistances by balancing two branches of the circuit, one containing a standard resistance and the other the unknown.
References: Frank A. Laws, Electrical Measurements, New York, 1938, pp. 165-69; Robert Bud and Deborah Jean Warner, Instruments of Science: An Historical Encyclopedia, New York, 1998, pp.663-65.
Astatic Mirror Galvanometer #10002
Electric Mfg. Co., Troy, NY
This is an example of an instrument that neutralizes the Earth’s magnetic field with a curved magnet above the galvanometer coil. William Thomson made the first version of this instrument in 1857. This example was patented in the United States on July 28, 1885. A silk fiber supports a short needle and a mirror at the center of a fixed coil. Judging by the three instruments in this collection, the Electric Mfg. Co. made instruments of very high quality and precision.
Reference: Gerard L’E Turner, Nineteenth-Century Scientific Instruments, 1983, pp. 201-2.
Wiedemann’s Galvanometer #10541 and 10294
Edelmann, Munich and Queen & Co., Agents
A ring magnet is suspended by a fiber on which there is a mirror to indicate the rotation angle. There are two adjustable coils, supplied with removable iron cores which slide in horizontal tracks to provide a variable sensitivity.
References: Queen Catalogue of Physical Instruments (1888), p.254; Max Kohl Catalogue No. 50 (c.1911) p.881; Whipple Museum Catalogue 8: Electrical and Magnetic Instruments, 1991, No. 13; Gustave Wiedemann, "Die Lehre von der Electricitat, 1895, pp. 306-309.
Silver Voltameter #10741
Hartmann & Braun, Frankfurt
An adjustable stand on a base with electrical terminals holds a vertical rod over a small platinum cup. At the end of the rod there is a double cone-shaped section that can be lowered into the cup. Voltameters are used to calibrate ammeters by measuring the amount of a metal ( usually silver) deposited in an electrolytic cell in a known time. The accompanying butterfly-shaped device is probably not part of the apparatus.
Reference: Max Kohl Price List No. 100 (c.1927), p.946