Galvanometers


Figure 1: General scheme of a galvanometer.

Galvanometer is the historical name given to a moving coil electric current detector. When a current is passed through a coil in a magnetic field, the coil experiences a torque proportional to the current. If the coil's movement is opposed by a coil spring, then the amount of deflection of a needle attached to the coil may be proportional to the current passing through the coil. Such "meter movements" were at the heart of the moving coil meters such as voltmeters and ammeters until they were largely replaced with solid state meters. 


 
Figure 2: Moving coil of the galvanometer.

The accuracy of moving coil meters is dependent upon having a uniform and constant magnetic field. The illustration shows one configuration of permanent magnet which was widely used in such meters. 


 

Figure 3: Operation of a galvanometer.

The galvanometer is used to measure very low currents, such as those in bridge circuits. In modified form, the galvanometer has the highest sensitivity of any of the various types of meters in use today. A simplified diagram of a galvanometer is shown in Figure 3. It is different from other instruments used for the same purpose because its movable coil is suspended by means of metal ribbons instead of a shaft and jewel-bearing arrangement often used in other instruments. The movable coil is wrapped around the aluminum frame of the galvanometer. The coil is suspended between the poles of the magnet by means of thin, flat ribbons of phosphor bronze. These ribbons provide a conduction path for the current between the circuit being tested and the movable coil. The ribbons allow the coil to twist in response to the interaction of the applied current through the coil and the magnetic field of the permanent magnet. They also provide the restoring force for the coil. Basically, the restoring force is that force necessary to return the movable frame to its resting position after a reading. The ribbons restrain or provide a counterforce to the magnetic force acting on the coil. When the driving force of the coil current is removed, the restoring force provided by the ribbons returns the coil to its zero position.


 

Figure 4: D'Arsonval
type galvanometer

Moving-coil Galvanometer. In this type of instrument, known also as the D'Arsonval form of galvanometer, the suspended system is a coil of fine wire which hangs in a strong magnetic field due to a permanent steel horseshoe magnet. In Figure 4 is shown a vertically placed horseshoe magnet, between the poles of which is hung a light rectangular coil of many turns of fine wire, the plane of the coil being parallel to the direction of the lines of force. The coil is suspended by a fine ribbon of phosphor-bronze which also serves to connect one end of the suspended coil to the outer circuit while the other connection is made through a spiral wound strip of the phosphor-bronze ribbon attached to the lower end of the coil. A cylindrical mass of soft iron is fixed midway between the poles of the magnet so that as the suspended coil turns its vertical branches move in the gaps between the core and pole pieces. This arrangement secures a strong uniform field, across which the wires of the coil pass, and when a current is sent through it, it is deflected. A small mirror mounted just above the coil and moving with it, enables the deflection to be determined by the telescope and scale, or reflected spot of light method. The moving-coil galvanometer has the advantage that it is not affected by changes in the earth's magnetic field, and can be used near dynamo machines and where there is considerable magnetic disturbance. Also the coil damps strongly or comes almost immediately to rest when the wires leading to it are touched together, forming a short circuit, as it is called. This damping is due to electromagnetic induction. 


 

Figure 5(A): 
Tangent galvanometer



Figure 5(B):
Forces in a
tangent galvanometer

Tangent Galvanometer. In the tangent galvanometer there is a circular coil having one or more turns of wire, at the center of which a magnetic needle is either balanced on a point or suspended by a fine fiber of silk or quartz. The instrument is placed so that the plane of the coil is vertical and in the magnetic north and south plane (Figure 5(A)). When a current is sent through the coil the needle turns to one side or the other, and the strength of the current is proportional to the tangent of the angle of deflection. The force due to the current in the coil is at right angles to the plane of the coil at its center and the strength of the field at that point in a given coil is proportional to the strength of the current (Figure 5(B)). Let G represent the strength of field at the center due to the coil when unit current is flowing, then IG will be the strength of field when the current strength is I. Let OA in Figure 5(B) represent the plane of the coil and O the point where the needle is placed, then when no current is flowing the needle points in the direction OA, being acted on only by the horizontal component H of the earth's magnetic force. The magnetic force F due to the current in the coil is IG and at right angles to H, therefore, the resultant force R is the diagonal of the rectangle whose sides are IG and H, and 

where x is the angle which the resultant force makes with H. But the needle must point in the direction of the resultant force, and so x is the angle through which the needle turns. Therefore 

and if H and G are known the current may be determined by measuring the angle x. In case of a tangent galvanometer the magnetic force F due to the coil is expressed by IG

But if the current is measured in electromagnetic units, 

And since the length of n turns of wire of radius r is,

The galvanometer coil constant G can be calculated from this formula when the coil of the galvanometer has so large a radius compared with the length of the needle that the poles of the needle may be regarded as at the center, and when the cross section of the coil is so small that all the turns bear ncarly the same relation to the needle. If G is determined in this way, r being measured in centimeters, and if H is found in C.G.S. units system, the current will be also found in C.G.S. electromagnetic units by the use of the formula: 

To obtain the current strength in amperes, we must take as the value of the coil constant:

By this method the strength of a current is determined in amperes directly from the fundamental units of length, mass, and time, for we have already seen how the measurement of H is based on these units. A tangent galvanometer in which the constant is determined in this way directly from measurements of the coil is known as a standard galvanometer.

Ballistic galvanometer is designed to deflect its indicating needle (or mirror) in a way that is proportional to the total charge passing through its moving coil or to a voltage pulse of short duration. Any conventional galvanometer may also be employed as a ballistic type, but the latter has smaller torque and higher inertia in the coil.


Pictures below show some examples of galvanometers used in the 19th - beginning of 20th centuries:
 
Figure 6: Pye Coulomb Balance

This unusual device balances out the attraction between the two balls and a magnetic field created by the horizontally mounted coil of wire. It is capable of measuring very small amounts of current.


 
Figure 7: Deprez D Arsonal galvanometer
"Item of the collection of Fons Vanden Berghen, Halle, Belgium".

 
Figure 8: Early Thompson Mirror Galvanometer
Manufactured by Elliott Bros., London.

This tripod-based all brass Thompson mirror galvanometer number 502 has overhead compensating magnets extending up from the top. This unit is the same kind as the one on Thomas Edison's desk while he was experimentg with early telegraph systems.


Thomas Edison using a Thompson galvanometer

 
Figure 9: Mirror Galvanometer
Manufactured by Knott Apparatus Company, U.S.A. 

The Ivory label reads: Designed for International Correspondence Schools, Scranton, PA. Manufactured by L. E. Knott Apparatus Company, Scientific Supplies, Boston, MASS.


 
Figure 10: Elliot mirror-galvanometer used as a sensitive receiever on undersea cables for telegraph connections.
"Item of the collection of Fons Vanden Berghen, Halle, Belgium".

 
Figure 11: Abraham Rheogram Prism Synchroscope

This extremely rare prism synchroscope has an early synchronous-motor driven horizontally rotating triangular prism and mirror system that provides the time-base for the light beams from the two low-mass mirror galvanometers. This provides a dual-trace plus time-base display. Original finished wooden case.


 
Figure 12: Galvanometer
Manufactured by Cambridge Instrument Co. Ltd, England.

A large beautiful complex brass galvanometer with hair thin meter needle, compensating magnet, and mechanism sticking up out of center of a large round brass base with leveling screws.


 
Figure 13: Paschen Type Galvanometer
Made by the Cambridge and Paul Instrument Company.

This company (originally the Cambridge Scientific Instrument Company) was founded in 1881 by Horace Darwin, Charles Darwin's youngest son, to supply the laboratories of Cambridge University. The Darwin family association with the company continued until the 1970s. The galvanometer was purchased for Caltech's freshman physics lab in the fall of 1921. It is a Paschen type--designed by the German spectroscopist Friedrich Paschen, based on the Thomson astatic galvanometer.


 
Figure 14: Early Gramme Ring Horizontal Coil Galvanometer

Very early and primitive horizontally mounted multi-tapped coil surrounding a horizontally oriented compass.


 
Figure 15: Tangent Galvanometer
Made by Philip Harris Ltd., Birmingham, England.

A large 4" diameter horizontally oriented compass indicator is located in the center of a 7" vertically oriented coil of wire all mounted on a wooden stand with leveling screws. The coil taps are marked 2,50,& 500. 


 
Figure 16: Tangent Galvanometer, Helmholtz type
Made by Philip Harris Ltd., Birmingham, England, ca. 1913.

The galvanometer is graduated in degrees of the horizontal rotation.


 
Figure 17: Tangent Galvanometer
Made by W.M. Welch Scientific Company, Chicago, U.S.; ca. 1930
(see acknowledgment)

The tangent galvanometer was introduced by Servais Mathias Poullet in 1837. Large diameter vertically mounted multi-tapped coil surrounding a horizontally mounted compass. The amount of current in the coil is proportional to the tangent of the angle of deflection of the compass. Such instruments would also be used to plot the horizontal component of the earths magnetic field in the laboratory-an important consideration in any laboratory where studies involving magnetism are to be conducted. The windings (20, 40, 80, and 160) are wrapped in a U-shaped channel in an 8 7/8" o.d. cast aluminum ring. Slots in the back of the ring allow the coil diameters to be measured. There is a 3 1/4" diameter chrome finished compass with raised polished metal numbers and divisions against a black plate, mounted on a center rod centered in the coils, the whole mounted in the center of an 8"diameter black painted wood base. Two chromed brass posts at the front of the base connect through contact plugs on a hard rubber rectangle on the base. An additional set of heavy coils are provided with separate binding posts at the back of the instrument. The entire instrument is mounted on a heavy cast brass tripod equipped with leveling screws. The second picture shows a slightly different version of the instrument.
Company description of this instrument.


 
Figure 18: Tangent Galvanometer
Made by Central Scientific Instrument Company.

The galvanometer is similar to the Welch galvanometer described above.


Close-up view of base

Close-up view of compass

 
Figure 19: Tangent galvanometer
"Item of the collection of Fons Vanden Berghen, Halle, Belgium".

 
Figure 20: Tangent galvanometer
Made by Siemens & Halske
"Item of the collection of Fons Vanden Berghen, Halle, Belgium".

 
Figure 21: Tangent Galvanometer
Made by Stoelting Company.

Very simple and primitive tangent galvanometer consisting of a compass mounted in a wooden block with a coil of wire wound around the block in a vertical plane.


 
Figure 22: Galvanometer for use in laboratory and for field testing used for early telegraph applications.
"Item of the collection of Fons Vanden Berghen, Halle, Belgium".

 
Figure 23: Galvanometer
Early General Radio Model 129.

The galvanometer uses two internal horseshoe magnets and shows signs of having been partly manufactured by hand.



A view of the early General Radio Galvanometer with cover removed.

 
Figure 24: Mirror galvanometer
Made by Sullivan
"Item of the collection of Fons Vanden Berghen, Halle, Belgium".

 
Figure 25:
Portable Pointer Type Galvanometer
Made by Leeds & Northrup Co., Philadelphia, U.S., before 1934.
(see acknowledgment)

The pointer galvanometer is a sensitive instrument for measuring current. It consists of a magnet coil with an attached needle suspended between the poles of a magnet. When an electric current is introduced into the magnet coil it induces a magnetic field, causing the coil to rotate in proportion to the current. The example here is a null instrument, that is it is designed to be used in circuits where two currents are balanced against each other, i.e. with a Potentiometer or a Wheatstone Bridge, until the meter reads zero.
Company description of this instrument.


 
Figure 26:
Reflecting Galvanometer, Current
Made by Leeds & Northrup Co., Philadelphia, U.S.,  ca. 1926.
(see acknowledgment)

The reflecting galvanometer is a sensitive instrument for measuring current. It consists of a magnet coil with an attached mirror suspended between the poles of a magnet by a thin gold ribbon above and a coiled gold wire below. When an electric current is introduced into the magnet coil it induces a magnetic field, causing the coil to rotate in proportion to the current. The mirror allows the user to see very small deflections, and thus to measure very small currents, by observing a distant reflected scale (one meter or more away) through a telescope. The instrument is mounted on an optional leveling base of cast iron.
Company description of this instrument.
Galvanometer Telescope,
Leeds & Northrup, Philadelphia, PA, U.S., after 1934.

A small, special purpose, telescope used to observe the scale as reflected by the mirror in a reflecting galvanometer.

 


 
Figure 27:
Reflecting Galvanometer, Ballistic
Made by Leeds & Northrup Co., Philadelphia, PA, U.S., ca. 1930.
(see acknowledgment)

The reflecting galvanometer is a sensitive instrument for measuring current. It consists of a magnet coil with an attached mirror suspended between the poles of a magnet by a thin gold ribbon above and a coiled gold wire below. When an electric current is introduced into the magnet coil it induces a magnetic field, causing the coil to rotate in proportion to the current. The mirror allows the user to see very small deflections, and thus to measure very small currents, by observing a distant reflected scale (one meter or more away) through a telescope. The ballistic galvanometer is specialized in having a wide coil characterized by a long period. It is designed to measure the quantity of electricity (charge) discharged through the instrument over a short period of time. Applications include the measurement of capacitance, inductance by comparison with a standard, and magnetic field intensities using a search coil. 


 
Figure 28(A): Reflecting Galvanometer, High Sensitivity, Type R
Made by Leeds & Northrup Co., Philadelphia, PA, U.S., ca. 1960.
(see acknowledgment)

The reflecting galvanometer is a sensitive instrument for measuring current. It consists of a magnet coil with an attached mirror suspended between the poles of a magnet by a thin gold ribbon above and a coiled gold wire below. When an electric current is introduced into the magnet coil it induces a magnetic field, causing the coil to rotate in proportion to the current. The mirror allows the user to see very small deflections, and thus to measure very small currents, by observing a distant reflected scale (one meter or more away) through a telescope. The instrument is 9" high with the base 5" in diameter. This particular instrument has a flat-black finish on the aluminum case, a heavy Bakelite base, and a chrome-plated knurled head for the clamping and adjusting shafts. A 70,000 Ohm critical damping shunt resistor is in place across the input. The mirror in this instrument is 3/8" dia, as opposed to the 1/2" mirror in the lowere sensitivity instruments described in the Cenco catalogs below.
Company description of this instrument.

Figure 28(B): Reflecting Galvanometer Scale with Lamp and Stand
Made by Leeds & Northrup Co., Philadelphia, PA, U.S., ca. 1960.
(see acknowledgment)

This is an accessory to go with the L & N Reflecting Galvanometers - it replaces a scale and telescope. In use the lamp projects an image of an arrow pointer which reflects off of the mirror of the galvanometer and back to the glass scale, thus providing a meter scale with a very long (one meter) effective needle length. This in turn gives the galvanometer a greater readability, and by keeping the mass and inertia of the actual moving element low, a greater sensitivity.
Company description of this instrument.


 
Figure 29: Galvanometer, Cat #3417
Made by Rubicon Company

 
Figure 30: Enclosed Lamp & Scale Galvanometer
Made by Leeds & Northrup Co., Philadelphia, PA, U.S., ca. 1960.
(see acknowledgment)
Company description of this instrument.

Acknowledgment: I would like to thank Professor Robert A. Paselk, Scientific Instrument Museum, Humboldt State University, for the kind permission to use pictures (#17, 25, 26, 27, 28a, 28b, 30) and the related texts adopted from his web site.