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wire or other suitable material under certain portions of the disk and in series with the lamp circuit, that the disk is differentially heated at the proper point; thus, with the peculiar armature construction described, a thermo-magnetic action is set up, somewhat akin probably to the action of the thermo-pile. The portion of the disk heated, is attracted toward the field coils, and thus (the disk being progressively heated and cooling rapidly owing to its thinness,) continuous rotation is set up. Sometimes in this construction the field is in series with the heating device and sometimes the heating device alone, is in series with the lamp circuit, and the field is either a permanent magnet or independently excited by a battery. Neither of these changes involve the principle of the meter, which is assuredly ingenious.

As in almost all other of the arts, electric meter construction shows the definite mark of evolution; thus, one class of meter cannot be distinctly separate from its neighbor, the one merging into the other.

The final form, and I think I may say the last form of thermo-meter at all worthy of description, is distinctly a clock meter also. The clock meters will come under our consideration some-what later in this paper, but this meter which I am about to describe can scarcely be separated from its kindred, the heat meters. Briefly, however, I may define the clock meter as one whose recording device or whose movement is actuated by an independent source of power, generally a clock, though sometimes a constant speed motor or other device.

I might add here, at the risk of digressing from my subject, that there is probably no known indicating device from the Cardew voltmeter to the Sir Wm. Thomson balance which could not be so applied as to make a more or less accurate clock meter.

The thermo-meter now under consideration is also a thermometer (a commentary perhaps on the English language.) An ordinary thermometer of somewhat modified construction, is surrounded at the bulb by a coil of wire in series, or in shunt series with the circuit to be measured. It is apparent that the rise and fall of this thermometer will be more or less dependent upon the heat developed by the coil, and the heat again will be more or less dependent upon the current passing, but, of course, the temperature of the surrounding atmosphere must necessarily introduce serious error.

Now, it has been more than once suggested, though never patented, I believe, that a constant ray of light supplied, we will say by an electric lamp burning when the meter is in circuit, if thrown upon this thermometer could be made to photographically mark on rotating sensitized paper, actuated by clock work at uniform speed, a line, or rather a solid block of light whose lower termination or line of demarkation would represent the height of mercury in the tube.

Thus, we have a curve of registration which only needs to be integrated with a planometer to determine the consumption, always supposing that the indicating device is correct. I have said, "only needs to be integrated." How desirable a meter would be in practical service whose record needs to be integrated can best be left to the judgment of the station manager and the public. It is largely a matter for personal experience and personal opinion. To compensate for external temperature it was suggested that the meter be so encased as to practically insulate it from heat or cold; but this is probably not feasible.

There is another device, or perhaps I had better say there might be another device for accomplishing the object of this last meter in a somewhat similar manner. The actinometer is probably familiar to all who have indulged in amateur photograhy; it consists of a piece of glass covered with small cubes, each cube of a more intense ruby red than the one beyond it, merging, in fact, from an almost clear glass to an almost perfectly non-actinic medium.

Now, it suggested itself to a certain electrician that if a number of these squares were arranged in a piece of glass, one above the other, and a lamp whose light should vary more or less directly with the potential on the lines, be placed before this glass or actinometer, a sensitive film being rotated behind the actinometer at a constant speed, the varying light of the lamp would draw a curved line, or rather a curved block of light and shade on the paper. This could be measured by a planometer to get the average voltage, or could be taken at points, to see what the voltage was at certain times; in fact, a recording. voltmeter. This device seemed very nice indeed It had only one fault-it would not work. I might say that the device was

my own.

At this point I think we may venture to dismiss the subject of chemical, thermo and similar meters, and give some attention to the clock meter.

The general character of the clock meter has already been briefly stated; that is, we have already defined the character of one form of clock meter. There is an almost endless variety of instruments of this kind. They are not all electricity meters by any means; some are recording voltmeters, others recording ammeters, and others have still different purposes. The familiar recording steam gauge is only a modification of this instrument. Many such meters have been designed for station work and for testing purposes, where great sensitiveness and considerable cost of construction were permissible. But these hardly come within the province of a treatise of this kind.

A strip of paper is almost invariably rotated over a drum, sometimes being fixed to the drum, and sometimes being drawn from one drum to another, the paper moving at uniform speed, and generally being divided into hours or other fractions of time by abscissæ lines.

This clock mechanism can be combined with any indicating device; it is only necessary to supply the connection which shall cause the indicator to mark the paper, and draw a crooked line. First attempts of this kind were generally made with ordinary solenoids, or sometimes with a simple coil and iron core rising and falling with the current. Whatever the character of the indicator or method of communication between the indicator and paper, it remained necessary to keep the friction of contact low.

First attempts were made with a pencil, bearing directly upon the paper, but the friction introduced by this device was fatal to accuracy. A glass pen has been substituted for the pencil with better results, but even this caused too much friction. The photographic method of line drawing is one of the best systems yet introduced, and is probably more familiar to the majority of us than the other methods, because it forms a component part of the Walker meter which has attracted attention at various times.

With an instrument of this kind a diaphragm is provided, attached to the indicating mechanism, and raising and falling with the variations of the indicator. Through this diaphragm is a small aperture, the best plan being to form this aperture by the crossing of two slits in two diaphragms. Through this small hole, light from a burning lamp is shed upon sensitized paper with the result already described.

One of the best methods of accomplishing the registration, perhaps, when all things are considered, the very best, is to at

tach to the pointer of the indicator, a steel point or needle with an iron armature mounted in the form of a spring, or in some similar manner. By placing an electro-magnet behind the paper to be marked, and sending through this magnet an electrical impulse at fixed periods of say, one, two or five minutes, the pointer is drawn sharply down to the paper, puncturing it, and is immediately released by the cessation of energizing current in the electro-magnet. Thus, the paper when removed, will have a continuous marking of punctures tantamount to a curved line. This device presents practically no friction, and is more or less simple and easy to carry out. The make-and-break necessary for the electro-magnet is easily actuated by the clock movements. But we may say of these forms, as well as of the forms of clock meter which are to follow, thas there is one serious objection to them all-they have to be wound up, which is certainly a fault. We may modify this statement by saying that many of these devices have an electrical attachment which makes them self-winding, and this is a step in the right direction at least. Whether it fully meets commercial necessities, is, I believe, an open question.

All of these instruments being curve-drawing meters, they cannot properly be considered as the thoroughly commercial article, which the successful electric meter must be. For, I think I am safe in assuming that to be successful the meter of to-day positively must have a direct reading dial, equally available for the consumer and supplier.

This brings us to a second form of clock meter of a preferable character. This class comprises those meters whose clock mechanism is accelerated or retarded by the strength of the current passing through them. There are several methods of accomplishing this, all of which apply the influence of the current to the pendulum of the clock.

For example, we have a pendulum with an iron end forming an armature which is attracted first to the right, and then to the left by two electro-magnets dependent for their strength upon the lamp circuits. This is an early and very ineffective form of clock meter, hardly to be considered as a practical affair, but typical of the class.

The most successful and probably the best form of meter of this kind consists of two clocks with a differential gear between. One of these clocks runs at a constant speed, the speed of the

other being governed by the circuit to be measured. At times by a pressure coil forming an armature, and a fixed series coil forming a field, or again, by a horse-shoe magnet pendent from the pendulum and vibrating back and forth above two coils in series with the lamps, the former being a wattmeter and the latter an ammeter. Both were designed by the same inventor. When nothing is passing through the meter, the speed of both clocks is the same, and the differential gear does not move; but when lamps are turned on, and the regulating device begins to act, one clock is accelerated or retarded according as the principle may be applied, and the difference between the constant speed clock and the varying speed clock indicates the consumption in directly read units.

I can safely say that this is an excellent meter, but is obviously open to the same objection as are other clock meters, since there are two clocks and the presence of the differential gear requires very nice adjustment between the two. There is another important factor here, which bears directly upon the question in hand, and that is, the cost of building meters of this class. The clock-work must not be of a cheap character, but must be good clock-work and good clock-work, is invariably expensive and as invariably delicate.

I might say, however, that the Aron meter, the typical meter of this type, proved itself so accurate at the recent meter competition in Paris, that it succeeded in dividing the first prize of 10,000 francs and a gold medal, with the Thomson recording wattmeter, the principle of which we shall consider later. Your attention may be directed to the fact that the Aron meter is a watt-hour indicator, something unusual, for almost everything considered up to the present point has been typically an ammeter or a modification of an ammeter.

Before turning to the final portion of this paper and considering the most popular and as yet the most successful class of recording instruments, viz. the motor meters, a little attention should be given to those clock meters whose principle is dependent upon the function of a cone or cones. There are a con

siderable number of meters of this class. It appealed to the inventive mind early, and our patent records are full of cone meters.

Briefly, the principle of all of these instruments is about as follows:-Let us assume a wheel rotating at a constant speed; let us assume a cone mounted on a shaft on which it is free to

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