Recognizing a Need: Long Scale Switchboard Instruments

Posted By Richard Jefferson on 19 December 2014

Posted in The Vintage Machinery Almanac

This article was originally published in Electrochemical and Metallurgical Industry Publication of June 1907. Information within this article is therefore correct as of 1907. The publication of this material aims to provide historical insight on the subject and its place in industry.

Recognizing the need for switchboard instruments well suited for use with generators of large capacity and where readings must be made at a considerable distance from the switchboard, the American Instrument Co. of Newark, N. J. has recently placed on the market a new long-scale instrument.

These instruments are provided with scales approximately 14 inches long, or twice the length of the usual large-size round-pattern instrument. Owing to this there is ample room for large divisions and large figures, which, together with the unique method of marking the scales make them extremely clear and legible. For this reason they are particularly well adapted for use with generators of capacities running up to several thousand amperes and also where it is essential that accurate readings be made from a considerable distance.

Perhaps the most unique feature of these instruments is the method of mounting them on the switchboard. When an instrument with a long-scale as described above is mounted entirely on the front of the board it projects a considerable distance, and is more  or less in the way, while if the same instrument is mounted flush it requires cutting a large irregular shaped hole, which weakens the panel very materially. This instrument projects less than 2 inches from the front of the board and at the same time only requires a circular hole 6 5/8 inches in diameter to be cut in it. Thus they are easy to mount, the panel is not unduly weakened and they offer all the advantages of the flush-type instruments. Very satisfactory illumination may be obtained by means of a lamp and bracket mounted over the instrument.

The internal construction of these new type-3 instruments conforms strictly to the high grade of excellence which the American Instrument Co. has already established in its round pattern and portable instruments. The magnets are of the very best magnet steel, aged and magnetized according to the latest and most approved methods. The mechanical construction of the moving coil, its mounting, etc. are extremely simple and rugged, so that they are well adapted for the hard service which is bound to come to any switchboard instrument.

Ammeters of this new type are arranged to operate in connection with the regular interchangeable switchboard shunts which are used with round-pattern switchboard instruments. These shunts are adjusted to give a uniform drop of exactly 50 Milli-volts on full load. Also the instrument, together with its leads, is adjusted to have an exact resistance of 1 ohm and to give full deflection on 50 Milli-volts. Thus it is clear that any shunt of any capacity can be used with any instrument plus any pair of leads, and correct results will be obtained. This feature is of greatest advantage, as it allows the use of any number of shunts of any capacity on one indicating instrument, the only requirement being that a suitable two-pole switch of negligible resistance be inserted in the leads between the instrument and shunts. Also should an instrument be disabled, through accident it can be returned to the factory for repairs and properly adjusted without disturbing the shunt at all; and while it is out of commission another 'American'' ammeter, whether of the same or different type, may be used and correct readings obtained, when the proper multiplier is used to make the scale values agree with the shunt capacity.

These long-scale voltmeters in common with all "American" voltmeters whether of switchboard or portable types are adjusted to have a uniform resistance of exactly 100 ohms per volt, so that a 150-volt instrument has just 15,000 ohms total resistance and a 300-volt instrument exactly 30,000 ohms resistance and so on. This uniformity makes it possible to use multipliers interchangeably should they be required, and also adapts the instrument for measuring insulation resistance and grounds most satisfactory.

Where generators of large capacity are used there will naturally be large currents flowing in the bus-bars, which will set up magnetic influences which affect the readings of the ordinary instrument. The American Instrument Co's long-scale instruments as well as its other switchboard round-pattern instruments are, however, provided with soft-drawn sheet steel cases, which provide a most efficient magnetic shield for the internal parts, so that powerful external fields do not influence the indications. As these cases are drawn into shape the material must be of uniform softness throughout. In consequence of this there is practically no danger of the cases becoming permanently magnetized as they do when cast iron is used. In addition to the shielding quality of these cases they are so designed that where parts come together there is ample bearing service, which effectively prevents the entrance of dust.

Mr. James G. Biddle. 1114 Chestnut Street, Philadelphia, is general sales agent for the American Instrument Co.

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