Ans. The essential parts are shown in [fig. 646]. The fixed coil A, composed of a number of turns of wire is fastened to a vertical support, and surrounded by the movable coil B of a few turns, or often of only one turn. The movable coil is suspended by a thread and a spiral spring C, below the dials which are fastened at one end to the movable coil and at the other end to a milled headed screw D, which can be turned so as to place the planes of the coil at right angles to each other, and to apply torsion to the spring to oppose the deflection of the movable coil for this position when a current is passed through the coils. The ends of the movable coil dip into two cups of mercury E, E', located one above the other and along the axis of the coils so as to bring the two in series when connected to an external circuit. The arrows show the direction of current through the two coils. An index pointer F is attached to the movable coil. The upper end of this pointer is bent at a right angle, so that it swings over the dial between two stop pins G, G', and rests directly over the zero line when the planes of the coils are at right angles to each other. A pointer H is attached to the torsion screw D, and sweeps over the scale of the dial. The spring is the controlling factor in making the measurement.
Fig. 646.--Diagram of Siemens' electro-dynamometer. It consists of two coils on a common axis, but set in planes at right angles to each other in such a way that a torque is produced between the two coils which measures the product of their currents. This torque is balanced by twisting a spiral spring through a measured angle of such degree that the coils shall resume their original relative positions. If the instrument be used for measuring current, the coils are connected in series, and the reading is then proportional to the square of the current. If used as a wattmeter, one coil carries the main current and the other a small current, which is proportional to the pressure. The reading is then proportional to the power in the circuit.
Fig. 647.--Diagram showing connections of Siemens' electro-dynamometer as arranged to read watts.
Figs. 648 to 650.--Wright demand indicator. This is a device for registering the maximum ampere demand of appreciable duration in any electrical circuit. It may be used on either direct or alternating current circuits. The essential features and principle are as follows: A liquid is hermetically sealed in a glass vessel consisting of two bulbs connected by a "U" tube, and a central tube called the "index" tube, connected to the upper end of the right hand side of the "U." Around the left hand or heating bulb, is placed a band of resistance metal, through which the current to be measured is passed, or a definite shunted portion of it. The heating effect of the current increases the temperature of the left hand bulb, causing the air to expand which forces the liquid up the right hand side of the "U" tube and into the index tube, where it remains until the indicator is reset. The height of the liquid in the index tube as shown by the scale, indicates the maximum current which has passed through the indicator. It is the difference in temperature of the air in the two bulbs which causes the flow of the liquid. Any change in external temperature causes equal effect in both bulbs and therefore does not affect the reading.
Figs. 651 and 652.--Weston illuminated dial station voltmeter and ammeter. The voltmeter has two indices, a pointed index for close readings and an index called the normal index, which enables a slight deviation from the normal voltage to be seen from a long distance. The "normal index" is inside the case and terminates in a circular disc of blackened aluminum. The disc is adjusted from the outside of the case by hand, by means of the knurled knob seen on the front of the case, so that it is directly below the point of normal voltage. When the indicating index reaches the point of normal voltage, the disc of the normal index appears in the center of the circular opening of the indicating index, a narrow ring of white being visible, encircling the disc of the normal index. The ammeter depends for its operation upon the fall of potential between two points of the circuit carrying the main current, and requires a difference of only about .05 volt to give a full scale deflection. When a maximum deflection is secured, the current passing through the instrument is never more than .07 ampere irrespective of the total capacity of the instrument. A separate shunt is used which is placed at the back of the switchboard. In many cases, a special shunt can be dispensed with and a short section of the mains on the switchboard, or the mains leading from the dynamo, can be used instead. On the basis of one square inch cross section per 1,000 amperes, a length of about 5 feet of copper conductor would be required as a shunt, in which case however, this section of the conductor must be adjusted with precision.