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galvanometer are, unlike those in the Thomson type, obtained by a special feature in the construction of the suspended coil. The latter, where thorough damping is required, is wound on a complete metal frame or
former of silver or aluminium, and the induced currents set up in the frame itself by its movement in the permanent magnetic field serve to check its oscillations, and cause the system to more rapidly assume the zero position, after having been deflected therefrom by the passage of a current through its coil. In the undamped form this frame or former is either divided at one point 80 as not to form a complete circuit, or is constructed of some non-conducting material such as ivory.
Messrs. Nalder Bros. have adopted 1-40th of an inch as the width of their standard scale division, instead of one millimetre, as is usual in such cases, their contention being that this dimension is a more convenient one for the purpose.
The "Figure of Merit” in their list is reckoned in megohms, i.e., the number of megohms through which an E.M.F. of one volt at the terminals of the instrument will produce a deflection of one scale division (1-40th inch), and the following particulars, excerpted from their list, apply to the various grades of this instrument supplied by them : List No. Figure of Merit.
150 Megohms Damped
Undamped Mr. Robert W. Paul, of 68, High Molborn, supplies the well-known Ayrton-Mather form of D'Arsonval galvanometer, which is depicted in the accompanying illustration, and is a very compact and useful form of instrument, occupying only some 7 ins. by 7 ins. by 8 ins. of space when packed in case complete for transit. It is supplied either in dead beat or ballistic form, as required, and can be obtained wound to any resistance from 5 to 1,000 ohms, the stock pattern having a resistance of some 325 ohms.
The salient feature, or suspended coil, of the instrument is self-contained, being constructed with a
rounding and protecting tube, which slides into attachments on the main body of the instrument, making contact simultaneously with the terminals thereof. These slides are complete in themselves, and are interchange able, so that a single instrument may be supplied with a comprehensive set of slides which will conform to a multiplicity of requirements. One of these detachable slides is depicted below, both in toto and with the inner coil tube removed and shown separately.
Coil Tube of Ayrton-Mather Galvanometer,
Coil Tube in Slide.
As mentioned above, the resistance of the stock pattern is 325 ohms wound in copper, although a winding of other material will be substituted if desired, and its constant is 20 millimetre scale divisions per microampère with a scale range of one metre.
The period of swing of the dead beat form is two seconds, and of the ballistic five seconds.
The accompanying illustration represents the New Model of this type of instrument, specially designed with a view to complete accessibility of working parts for laboratory use.
Mr. James Pitkin, of Clerkenwell, supplies a cheap form of the Holden-D'Arsonval galvanometer, which is an improved form of dead. beat instrument designed by Major Holden, R.A. It is depicted below with the case removed, and, as regards its detailed construction, I cannot do better than quote its description as issued in brief pamphlet form by the maker :
"The success which has attended the introduction of the Holden-D'Arsonval galvanometer in 1887 has led the inventor, Major Holden, R.A., to turn his attention to an instrument which, while being of such a price as to be within the reach of students and others requiring a galvanometer of the reflecting type, should yet have many
Holden-D'Ar al Galvanometer, by Pitkin. of the advantages of the more expensive form. The engraving shows the instrument, which is about 8 ins. high, with the cover removed, revealing the interior. The base is a brass casting, into which the levelling screws turn. This casting carries on the upper surface the
stamped steel magnets, which produce the field in which the coil moves, and it also carries the tubular socket into which a frame carrying the suspended coil fits. The removable frame forms one of the features of this galvanometer, as it enables the coil to be changed instantly with the greatest ease, so that the galvanometer may be used for measurement of the smallest currents, from millionth of an ampère to 1,000 ampères, either with continuous or alternating current, without removing it from the testing table.
“ The frame consists of two parts, insulated from one another, the lower being, when in position, in contact with the base of the instrument. The coil is suspended from a torsion head at the top to a tension spring below. The soft iron core, around which the coil moves, is attached to a cylindrical plug on the frame, which fixes into the socket before mentioned. A flexible wire connects the upper insulated portion of the frame with one terminal, the other being in contact with the base. Different descriptions of frames are supplied. For instance, for zero testing there is one of 500 turns, of about 500 ohms resistance, while for measuring ampères, with the instrument shunted by means of a platinoid or manganin strip, the coil is wound with only 10 turns of the same material as the strip is composed of, and will give a deflection over the whole of its scale with .002 ampère; then to measure, say, 100 volts with the above coil, resistance of 50,000 ohms is provided, wound on two bobbins, each being 25,000 ohms; so that by using the two in series the instrument is suitable for measuring 100 volts; with one in series, 50 volts; with two in parallel, 25 volts. There is no temperature allowance, owing to the extremely small current and to the materials used.
Another frame for the measurement of alternate currents is based on the expansion of a wire under the heating influence produced by a current passing through it. This form is most useful for measuring voltages with an added non-inductive resistance of about 150 ohms per volt to be measured. As regards figure of merit, one volt through one megohm gives a deflection of 10 millimetres, using the 500 turn coil at a distance. of two metres from the screen. When required, the
instrument can be made more portable by means of a patented clip, fitted to the coils, so that the coil may be firmly fixed without loosening or slackening the suspension, thus enabling them to be subjected to the roughest usage, and even sent by post without fear of breakage.”
The above instrument is also constructed in a somewhat more elaborate form, under the name of the HoldenD'Arsonval Universal Reflecting Galvanometer. The principle of this instrument is precisely similar to that described above, the only difference being that it is somewhat more sensitive, and commands, in consequence, a slightly higher market value. The standard pattern is provided with four interchangeable coils and frames, viz. :-(1) A coil of 500 turns with a resistance of 600 ohms ; (2) A coil of 250 turns and 265 ohms; (3) A coil of 125 turns and 133 ohms; (4) A coil of 250 turns and 70 ohms resistance.
The constant for the 500 turn coil is ten millimetre scale divisions with an E.M.F. of one volt through ten megohmg, the length of the scale range being two metres.
We now come to the productions of Messrs. Elliott Bros., which are of various types, calculated to meet a variety of requirements. The simplest and cheapest form of D'Arsonval galvanometer manufactured by this wellknown firm is illustrated below, and consists of a simple suspended coil and permanent magnet mounted in an