ratory work. One of the simplest forms is Kelvin's vertical electrostatic voltmeter, consisting of two fixed metallic plates B, between which is pivoted a movable metallic plate, A, all three being parallel

and vertical. The pair of fixed plates and the movable plate are respectively connected to the conductors between which the vcltage is to be measured. This causes the plates to become oppositely charged; and the movable plate is deflected in its own plane against the force of a weight attached to its lower part, and the position of the pointer C is read on the scale E. The attraction. is proportional to the square of the potenial difference; hence electrostatic instruments have a disadvantage similar to that explained in the case of electrodynameters. In fact, it is very difficult to obtain a practical electrostatic device which will measure pressures of less than about 50 volts. But in electric lighting the potentials used are very rarely less than 100 volts, and are often as high as 2,000 volts or more; consequently these instruments can be adapted to that purpose.

The advantages of electrostatic voltmeters are:

1. They are applicable to either alternating or direct currents. 2. They are extremely simple in construction and action. 3. They are cheap.

4. Their accuracy does not depend upon any condition which is likely to change.

5. They consume no energy, since no current flows through them.

6. They are absolutely free from magnetic disturbance.

7. They can readily be made for measuring potentials up to 25,000 volts, or even more.

The last advantage is so decided that this type would seem best for very high potentials, and it possesses the six other advantages enumerated, even when employed for lower voltages. Up to the present time it has not been developed to the extent that its merits would seem to warrant. In practice, it is customary to measure high voltages by using a potential transformer, which reduces the pressure in the ratio of 10:1 or 100:1, so that an ordinary voltmeter can be used.

Electrochemical Instruments are not commonly used for practical measurements, almost the only example being the Edison electrolytic meter, described in Volume II. It was used in large numbers to measure the current supplied to the various customers, but has now been replaced by mechanical meters.

Special Alternating-Current Instruments.--Forms of ammeter and voltmeter have been devised in which effects peculiar to the alternating current are utilized. For example, the action of repulsion, discovered by Professor Elihu Thomson, may be applied to purposes of measurement. These have no great advantage over the ordinary electromagnetic and electrodynamic effects, and are not only limited to alternating currents, but also in most cases depend upon the frequency.

The Thomson Inclined-Coil Meter of the General Electric Co. is extensively used for alternating-current measurements. The working parts of the ammeter of this type are shown in Fig. 202. A coil A, through which flows the current to be measured, is mounted with its axis inclined as shown. A vertical spindle mounted in jewel bearings and controlled by a hair-spring passes through the coil; and to this spindle are fixed a pointer b and a vane of thin

sheet iron a. This vane of iron is mounted obliquely to the spindle. When the pointer is at the zero-point of the scale, the iron vane a lies nearly across the axis of the coil; and when a current passes through the coil, the vane tends to turn until it is parallel to the axis of the coil, thus turning the spindle and moving the attached pointer over the calibrated scale.

T

Fig. 202. Movement of Thompson Inclined-Coil Meter.

The inclined-coil voltmeter is identical with the ammeter, except that the former has fine wire in the fixed inclined coil, and in place of the iron vane, a coil of fine wire is employed, placed in series. with the fixed coil, and with an auxiliary, non-inductive, high resistance.

Wattmeters. If the Siemens electrodynamometer described on page 449 is modified by winding one of the coils with a great many turns of fine wire, connected to the circuit so that the current received by it shall be proportional to the voltage, the other coil being, as before, in the main circuit, and having the total current passing through it, the result is that the mutual action of the two coils depends upon both the pressure (volts) and current (amperes) of the circuit. In other words, the force exerted between them will be directly proportional to the product of the voltage and current, which is the number of watts.

For direct-current purposes a wattmeter is not so very important; since all that is necessary is to read the potential from the voltmeter and the current from the ammeter, and multiply the two numbers together to obtain the power in watts. But in alternatingcurrent measurements the case is different; because there is a lag of the current wave with respect to the E.M.F. wave, if the circuit contains self-induction, as it almost always does. On the other hand, the effect of capacity is to produce a lead of the current; but

in either case the "apparent watts" obtained by multiplying the volts by the amperes, as indicated in different instruments, is not the real energy. This must be multiplied by the power factor, which is the cosine of the angle of lag, in order to find the "true watts." (Vol. II, p. 119.) It is difficult to determine this power factor; hence it is far more convenient to employ a wattmeter, which eliminates the error due to lag or lead so that the actual power may be read directly.

Recording Volt-, Ampere-, and Wattmeters.-These are employed in electrical generating plants to make a continuous record of the output in volts, amperes, or watts. For example, a pen or other marking-device is attached to the indicating part of a voltmeter, and a strip or circle of paper is slowly moved by clockwork so that a line is traced upon it showing the voltage at any time. These instruments require more positive action, and therefore more energy, than the ordinary visual meters that merely move a pointer

Ampere- and wattmeters are also made which integrate or sum up the total number of ampere-hours or watt-hours during a given period. These are chiefly used to measure the current supplied to each consumer, and they will be described under the head of Recording Meters in Volume II.

For further information on electrical measuring instruments, the reader is referred to:

Electrical Engineering Measuring Instruments, by G. ASPINALL

PARR.

American Meter Practice, L. C. READ.

Measuring Instruments for Switchboard Use, a paper read before the Institution of Electrical Engineers, London, by K. EDGCUMBE, July, 1904.

CHAPTER XXIV.

LIGHTNING-ARRESTERS.

THE various devices employed to protect electrical apparatus from lightning or atmospheric electricity are commonly termed lightning-arresters. This name, however, is not very appropriate; since they do not in any sense stop the discharge, but merely act to divert it, and convey it harmlessly to the ground.

Lightning-arresters are required wherever an electrical circuit, or any portion of it, extends out-doors; but they are obviously unnecessary if the conductors are wholly in-doors, underground, or submarime. The liability of an aerial wire to receive discharges of atmospheric electricity depends upon its length, its height above the ground, and its location, that is, whether it runs over hills or mountains; and certain regions are far more subject to this trouble than others. In England, for instance, its occurrence is comparatively rare and insignificant, whereas in the Rocky Mountains it is a matter of serious and almost constant difficulty.

The troubles which are likely to be caused in an electrical plant by atmospheric electricity are:

1. Puncturing or charring of the insulation of the electrical machines, instruments, or conductors.

2. Melting off of wires, or fusing together of contact points. 3. Danger to persons.

4. Liability of starting a fire.

The first or second of these accidents will often almost ruin an electrical machine or instrument, and involve considerable time and expense in repairing. The last two, although of less frequent occurrence, may be even more serious in their results.

In many cases where a circuit is partly overhead and partly underground or under water, there is danger that the former portions will receive atmospheric discharges, which, running down