Ammeters

   The analogue ammeter is a basic meter movement with a shunt placed in parallel across it. The movement goes full scale with only a milliampere or so of current through it, and the shunt passes the extra current around the meter movement. The fraction of the overall current which passes through the movement is a function of the resistance of the movement and the much lower resistance of the shunt. The basic mechanism is that developed by Edward Weston in the last few years of the 19th century. 

   That being said, almost all 20th century ammeters look alike. Here are three meters which have unique cases. At the right is a Current Indicator made by Whitney Electrical Instrument Company of New Hampshire; the earliest patent date is May 16, 1893. This instrument is in the Kenyon College collection. 

   The two ammeters below are in private collections and date from the early years of the 20th century..


                             Richard Zitto                                                                            Thomas Greenslade

   And then there is the Volt-Ammeter. This probably has an internal shunt for use as an ammeter and an internal multiplier for use as a voltmeter. The two instruments below have an unusual upright configuration on a horizontal base. The one at the left, by the Ziegler Electric Co. of Boston is in the museum room at the Physics Department of Washington and Lee University; the example by Knott on the right appeared in a eBay auction.
 

  The small meter at the left is in author's collection. He bought it at a yard sale near Boston ca. 1985 for only a dollar or two. 

   The 1916 catalogue of the L. E. Knott Apparatus Co., Boston, describes this as a "Horizontal Galvanometer, D'Arsonval movement, jeweled bearing, 0 center. This is a commercial type of instrument in horizontal or laboratory form. The range is such as to make it of the greatest value in general laboratory practice. Quick action, quick reading, adapted to a wide range of experiments, such as Induction, Polarization of Cells, Measurements of the Wheatstone Bridge, where an accuracy equal to ½ millimeter on the bridge is considered sufficient. Owing to the form of the pole pieces the scale is proportional to the amount of current going through, thus giving the instrument a range of usefulness far grater than its sensibility would indicate .....................................$7.50"


   This little galvanometer in the Greenslade Collection is only 10 cm high. It was made by the "Thompson-Levering Co., Makers of Scientific Instruments, Philadelphia, Pa." and is marked on the top, in ink, "2 µA/div", with 10 divisions on either side of zero. 

   A very similar instrument was sold by Leeds and Northrup of Philadelphia. In their 1907 catalogue this is listed as a portable d'Arsonval galvanometer and priced at $20.00. The sensitivity is the same as the Thompson-Levering instrument. 

The range of a basic ammeter movement may be extended to lower values by adding low-resistance shunts across it. These shunts by Weston Electric probably date from the nineteen twenties. The current leads are the heavy connections at the top, and the ammeter leads are the black ones at the bottom. These shunts were probably used with the ammeter below.


 
   The large Weston laboratory standard ammeter at the left was probably used with the shunts shown above. This meter includes a thermometer for temperature corrections, a mirrored scale and a bubble level. 

   It is retired from the laboratories at the University of Texas in Austin.



 
 
 

   This second example of a "Weston Direct Reading Laboratory Standard Milli-Volt Meter" is at Westminster College in western Pennsylvania. 

   It has an 1890 patent date, is 40 cm square, and is serial number 621,


   This early ammeter, dating from the very beginning of the twentieth century, is included because its wood has acquired a very nice patina over the years. The case is marked "Keystone Electrical Instrument Co., Philadelphia, Pa, DC Milliammeter" and the writing on the scale says "Made for the Central Scientific Co., Chicago, Ill." 

   It is in the collection of historical instruments at Kenyon College in Gambier, Ohio. 


   The  meter at the left has a basic galvanometer movement. On the front panel are shunts to enable the meter to measure currents up to 25 A and multipliers to allow potentials up to 125 V to be measured. 

   The 1916 catalogue of the C.H. Stoelting Co. of Chicago notes that "this instrument is designed for the lecture table, but is equally well adapted for students' use, having all its mechanism, internal and external, exposed to view. The movement employed in it is of the well known Weston Standard patented movable coil type." 

   A damping resistor must be permanently connected, as the pointer will come to rest without oscillations. 

   This instrument is in the Greenslade Collection.


   This massive ammeter, with a thick, cast-brass front, has a full-scale reading of 44 Amperes. The lower portion of the scale is non-linear, suggesting that it was used to measure alternating current. On the front is cast "Fort Wayne [Indiana] Electric Works",  "'Wood' Am-Meter" and "Pat'd Oct 8, 1889;  May 22, 1894". 

   It is in the Greenslade Collection.



 
 
 

   The handsome ammeter at the left was made by the American Instrument Company of Newark, New Jersey. It bears patent dates from 1906 and 1907, and is in the collection of Westminster College in western Pennsylvania. 

   Its twin is on the voltmeter page.



 

   The device at the left, made by the Brush Electric Company of Cleveland, Ohio, is a completely different approach to the problem of measuring electric current. 

   Here, the current passes through a pair of coils. The magnetic field, and hence the magnetic force of attraction for the pair of soft-iron rods, is proportional to the magnitude of the current. 

   The apparatus is at Case Western Reserve University in Cleveland.



 
 

   At the left is another unusual ammeter from Case Western Reserve University. This was made by the Edison General Electric Company of Schenectady, New York. 

  The curved iron wire is drawn up into the curved solenoid when current passes through the coil. The needle is attached to the wire at the same point as the wire's pivot. The scale is reasonably linear.

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