an area which represents the total quantity of electricity in ampere-hours which has passed into the house. An instrument which will record automatically this quantity is called an ampere-hour meter. Again, let the vertical lines drawn in a similar diagram represent the power in watts delivered at each hour to the house; the area of the curve then delineated represents the total electric energy in watt-hours which has passed into the house. Any instrument which will automatically record this energy is called a watt-hour meter. Instruments for the measurement of electric energy or quantity are generally cailed simply meters, and they are classified into ampere-hour meters and watt-hour meters. A more detailed classification of all the different forms of meter already invented would be a rather difficult thing to make on a perfectly correct basis. An approximate classification of meters for the measurement of alternating currents may be made as follows: 1. Graphic recording ampere-hour meters. 3. Continuously recording ampere-hour meters. Of the first two classes the Holden ampere-hour and Mengarini watt-hour meter are good examples. In these instruments an arm carrying a pen is displaced over a paper-covered drum, which is revolved uniformly in 24 hours by a clock. The motion by which the pen is displaced is regulated by a part of the instrument which is simply an ammeter or a wattmeter, and the displacement of the pen is proportional to the current or the power passing through this measuring part. When, therefore, the diagram is cut off and unrolled, we find on the paper a curve which represents, by its ordinates, either the power or the current at any instant; and, if the whole area of the curve is integrated, then such area represents the whole energy or quantity which has passed through the meter in 24 hours. These instruments have the advantage, therefore, that we practically record two quantities at once; and they serve two purposes, of indicating the instantaneous current or power, and the total current quantity or energy, but they have the disadvantage that they are not self-integrating. Of the continuously recording ampere-hour meters there are two well-known types in use, respectively for recording alternating current quantity and continuous current quantity. These were invented by Shallenberger and Ferranti. Taking first the Shallenberger alternating ampere-hour meter, it is constructed as follows. It consists (see Fig. 111) of a small transformer, one coil of which we may call the primary, and which is in series with the circuit in which the current to be measured is flowing. The core of this transformer is a little soft iron disc, which is capable of revolving on an axis. This axis. is geared at the top with a counting mechanism which records the number of revolutions of the disc, and at the bottom there is a vane or fan of thin aluminium which serves to retard the rotation of the disc. The secondary circuit of this transformer (see Fig. 112) consists of a small coil of copper, which is closed upon itself, and which is placed with its axis. inclined at 45° to the axis of the primary coil. When the primary current flows through the primary coil it does two things-it magnetises the core, and it induces a secondary current in the closed secondary circuit. The phase of this secondary current is about 90° behind the phase of the primary current, and thus the magnetism of the iron core, which is in a direction at right angles to the plane of the primary coils, also lags in phase behind the primary current by about 90°. The magnetism of the core and the induced secondary current are, therefore, in step, and are in such directions that the axis of the disc is always being pulled round by the induced field of the secondary coil. If, then, there were no friction of any kind, the iron disc would be therefore continually accelerated in speed, but since the air opposes a resistance which varies approximately as the square of the velocity, and since the mean driving force is proportional to the mean square of the current strength, it follows that the total number of revolutions which the disc makes in any given time is proportional to the total mean quantity or ampere-hours which have passed the primary circuit. The meters can therefore be calibrated by a constant in such a way that they read directly on counting dials ampere-hours, and if the pressure between the mains is kept constant, they may be graduated to read in Board of Trade units. These meters are very simple to construct and very fairly accurate in performance, and they have therefore come into extensive use. The velocity of the disc being at any time proportional to the mean current passing through the meter, we can, if the current is kept tolerably constant, employ the instrument as an ammeter. By moving the position of the secondary coil a little adjustment can be made in the meter for change of frequency, and the meter can be calibrated for the particular frequency for which it is intended to be used." The Ferranti continuous current ampere-hour meter consists of an electromagnet, the coil of which is of mild steel with a certain retentivity for magnetism. The core is worked at such a low flux density that the magnetisation of the core is always nearly proportional to the strength of the magnetising current. This electromagnet has a disc-shaped cavity in the core (see Fig.113) lined with an insulating material, and which is filled with mercury. The main current flows through the electromagnet coils, and then entering the mercury at the periphery of the disc-shaped cavity, which is not insulated, flows radially inwards in all directions to the centre of the mercury, whence it goes to the terminal of the meter. Under these conditions the mass of mercury is set in rotation, and its rotation is retarded by radial grooves which are formed on the sides of the chamber. The force driving the mercury is proportional to the square of the strength of the current, and the force retarding the rotation of the mercury to the square of the speed. Hence it follows that the number of rotations in any given time. is proportional to the total quantity of electricity which has passed. The rotation of the mercury is communicated to a counting mechanism by means of a little vane F immersed in it. The counting mechanism is so devised that the dials read electric energy in Board of Trade units on the assumption that the voltage of the circuit remains constant. The two meters last described, viz. the |