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or receding the like poles of these magnets to or from one another, the resultant field is increased or diminished, and its consequent controlling effect upon the needles thereby adjusted. Needless to state the iron case must be free from magnetism in itself. Where a solid iron case is not available, a hollow shell filled with iron filings answers the same purpose, and is, with a little ingenuity, easily constructed.

These systems of magnetic control and shielding are not, of course, necessary with the moving coil type, which instruments are immune from external magnetic influences.

The Electrometer.-An electrometer is essentially an instrument for measuring or comparing difference of potential by the applied principles of electrostatic attraction and repulsion; and, further, its action is amenable to the same laws which control the latter properties.

An absolute electrometer is one which records its measurements directly in absolute units without the necessity for comparison with measurements taken in another apparatus, or, in other words, the results obtained on it are in terms of other known units, such as the physical reactions of springs and torsional wires.

A B, and C D, Fig. 7, represent two parallel plates separated by a distance a, which is neglibly small in comparison with any plane dimension of the plates. Such an arrangement of parallel plates with a separate portion in the centre of one of them, A B, is met with in the case of the "attracted disc" electrometers with what is known as the "guard-ring," corresponding to that portion of the disc A B surrounding its central area.

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Sir William Thomson, who has designed almost all the electrometers adopted for present-day use, has divided them under three heads, viz.: (1) Repulsion Electrometers, such as the original torsion balance of Coulomb; (2) Attracted Disc Electrometers, exampled in the SnowHarris, Thomson's Absolute and Long Range Electrometers; and (3) Symmetrical Electrometers, to which

belongs the well-known and widely adopted Thomson Quadrant Electrometer, so called from the form of its conductors. I shall not attempt to deal with all the above-mentioned types of instrument, but shall select as my model that popular and widely adopted specimen of the Symmetrical class known as the Quadrant-Electrometer. It was evolved, from a primary contrivance known as the divided ring instrument, the principle of which is diagrammatically represented in Fig. 8, where A, B, represent two flat semi-circular strips of metal separated at their two extremities by a small insulating gap. Passing through the centre of the circle enclosed by these strips was a vertical suspension wire, carrying on one side of it, and at right angles to itself, a light needle C. It is obvious that if this needle be charged with electricity of a certain sign, say positive, and so insulated that its potential remains constant, and also if the two strips A and B be charged with positive and negative electricity respectively, the one will repel and the other attract the needle C, and the force of that attraction and repulsion, as measured by a torsion head

A

B

FIG. 8.

acting on the suspension wire, will be an indication of the difference of potential between the semi-circular strips A B. In the quadrant electrometer the semi-circles developed into four box quadrants A, B, C, D, Fig. 9, of which the opposite quadrants A, B, and C, D, are respectively connected together. The needle is also made symmetrical about its axis, and takes the form shown by the dotted lines in Fig. 9, which is a plan and elevation of the needle and quadrants as normally arranged. The quadrants themselves are mounted on insulating stems suspended from the cover, one of them being so arranged

as to be adjustable micrometrically. The needle, which is of thin sheet aluminium, is mounted horizontally on the vertical suspension wire which ends at its upper ex

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tremity in a small cross-bar, to the two equi-distant extremities of which is attached the bifiliar suspension; just below the cross-bar is attached a concave mirror protected from external influences by a split tube. At its lower extremity the suspension wire ends in a small platinum weight, suspended in sulphuric acid which has been boiled with sulphate of ammonia to expel deleterious impurities, and thus exercises a damping effect upon the suspended system. The bifiliar suspension is attached to an arrangement of screws and springs permitting of all the necessary adjustments for azimuth, equality of suspensory length, etc. The outside coating of the glass case, which takes the form of an inverted bell jar, is constituted by strips of tinfoil pasted on it, whilst the inside coating is represented by the sulphuric acid, which also serves to keep the interior of the apparatus free from moisture. The platinum weight at the lower extremity of the suspension wire is submerged, as before mentioned, in the sulphuric acid, and thus connects the needle with the interior coating, whilst the quadrants are provided with suitable electrodes for connecting them either with the case, or the body under test at will.

The mirror and suspension adjustments are enclosed by a protecting cover, known as the "lantern," in the side of which an opening is left, closed by a glass window, to permit of the mirror being used as in an ordinary reflecting galvanometer with lamp and scale. The instru

ment is provided with a gauge and replenisher, whilst the suspension wire is protected from external influences by an encircling guard-tube connected to the acid by a platinum wire at its lower extremity.

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Sir Wm. Thomson's (Lord Kelvin) Quadrant Electrometer,

An induction plate, or sheet of metal slightly smaller in area than the upper surface of one of the quadrants, but of similar shape, is supported from the lid of the instrument by an insulating glass stem, and is provided with a terminal or electrode communicating with the exterior.

As the instructions for setting up the quadrant electrometer vary slightly with certain variations in the details which have been made from time to time, and are supplied with the instrument, the writer does not consider it necessary to repeat them here, but will proceed at once with a description of the method of using the instrument.

To charge the containing jar, the main electrodes and the induction plate, together with a binding screw on the cover, are electrically connected together, and the charge imparted by a small electrophorous supplied with the electrometer, to the charging electrode or terminal, being

subsequently adjusted to the requisite normal as indicated on the gauge, by means of a replenisher.

Under normal circumstances leakage of charge is inappreciable, causing, so to speak, a hair-breadth variation in the gauge indication per 24 hours, and may be restored by the replenisher. Should the leakage prove excessive, however, proceedings must at once be taken to eliminate it. To this end, the glass insulating stems should be well washed with a piece of hard silk ribbon dipped in soap and water, being subsequently rinsed to remove the soap, and finally dried by friction with a dry piece of the same ribbon. Similarly, the various parts should be dusted with a fine camel-hair brush, and the glass jar itself, in the event of any accidental splashing of the acid, well washed and dried. The sole plate of the replenisher, a frequent source of leakage, can be treated by removal and immersion in boiling water, followed by a subsequent drying and the application to its surface of a paraffine film. Similar treatment is suitable for other ebonite fittings, such as the supports for the electrodes.

Although there are, of course, a large number of instruments other than those described in the preceding paragraphs under the heading of galvanometers, the types thus enumerated will be ample for the tests to be described in the following pages; and this being the case, the writer will not risk confusing the minds of his readers by entering into the details of other patterns.

Shunts. In utilising some of the more delicate types of galvanometer just described, it will be obvious that the passage of a current of too great magnitude through the winding would tend to produce a somewhat violent throw or deflection of the moving system, which would, besides having an injurious effect upon the delicate suspensions, be unreadable, owing to its indications extending beyond the extreme limits of the scale. In order, therefore, to bring such abnormal currents within the practical working range of the instruments in question, it is necessary to adopt some method by means of which their ultimate effects upon the suspended system can be materially reduced, and this object is attained by the use of what are technically known as "shunts."

Referring to Fig. 10, if G be a galvanometer, through the coils of which is passed a current of electricity by way of

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