the swing coil of the Wattmeter is quite free to move. Care should be taken to run the "leading in and "out" wires carrying the main current to the Wattmeter, close together or twisted. Also the main wires of the rest of the circuit close together in order that the current flowing in them shall exert magnetic influence on any of the instruments. (2) R being at its maximum value, close S, and adjust R so as to obtain about 1th of the full load current through W, the pressure being maintained at exactly 100 volts by varying the carbon rheostat (Rh). Note the readings of all the instruments. (3) Repeat 2 for about ten different readings on W rising by about equal increments to the maximum current allowable, and calculate for each the percentage error of the Wattmeter and the mean. Tabulate as follows (4) Plot a curve having values of (W) as abscissae and the corresponding Wattmeter readings (D) as ordinates. (18) Calibration of a Wattmeter with Alternating Currents. (Three-Voltmeter Method.) Introduction.-Wattmeters form a class of measuring instrument the chief application of which consists in measuring accurately the power taken up in alternating current circuits. The great value of a Wattmeter in such measurements practically disappears with direct currents as the individual factors of power, namely "volts" and "amperes," are usually here required, and in addition the product of the two can easily be obtained and at once gives the "true power." With alternating currents, however, this last remark is not true, and herein lies the great value of the properly constructed Wattmeter, in that it measures the true power in such a circuit. For it to be capable of doing this, however, it must be carefully constructed, and there must be no iron and preferably no other metal work near the coils. Wattmeters when used on alternating current circuits are liable to the following sources of error: (a) owing to the fine wire coil possessing some self-induction and consequently impedence, the current in it is not able to rise to the same maximum strength which it would do for a direct P.D. of similar magnitude; (b) this impedence causes a lag in phase of the current in the fine wire coil behind the P.D. across which it is placed. 66 (c) A third source of error common also to all voltmeters, and occurring both with direct and alternating currents, is that due to the alteration of the resistance of the fine wire coil due to change of temperature, and which can be minimized in the manner described later on. From the preceding remarks it will therefore be evident that when a so-called "Non-inductive Wattmeter" is calibrated with direct currents (which is usually the case) its constant so obtained will not be correct for alternating currents. The instrument will also read differently for variation of the "frequency" of the current even though the actual power being measured remains the same. Thus a Wattmeter may with advantage be calibrated with alternating currents on a circuit having the same "constants," namely voltage, frequency and wave form," etc., as that in which it is eventually desired to measure the power. The calibration can be performed by what is commonly known as the 3-voltmeter method of measuring power in alternating current inductive circuits, and by it the " 'true power may be obtained with almost any degree of accuracy desired by using an accurately calibrated voltmeter and by repeating the observation two or three times, noting the mean. It has the advantage that only one alternating current voltmeter is required, though three similar ones may be used if available. Apparatus. Alternator D and its exciting circuit (not shown) A D S W P400000000000 b V V FIG. 18. under rheostatic control or other convenient source of alternating current supply; inductive portion PQ of the circuit in series with a strictly non-inductive portion QR; two 2-way keys k, k; Cardew or low-reading electrostatic voltmeter V accurately calibrated; main switch S; Wattmeter W to be calibrated ; alternating current ammeter to indicate the current merely for reference only. Note. The resistances of PQ and QR should both be fairly small compared with that of the voltmeter V. 66 ་ Observations. (1) Connect up as in Fig. 18, and adjust the pointers of all the instruments to zero, levelling such as need it. See that all lubricators in use feed properly, and then start D. (2) Adjust the speed of D so as to obtain the desired frequency," say 100 per sec., at the same time varying the excitation to get the proper voltage, suppose 100 volts across PR. Adjust the resistance of PR so as to pass about th of the full load current (necessary to give a full scale reading on W) through Then the speed and voltage being constant, note the reading on A, W, and in quick succession the voltages V1, V2 and V across PR, PQ, and QR respectively by moving k1 and k2 simultaneously. W. (3) Repeat 2 for about ten different currents rising by about equal increments to the maximum allowable. (4) Calculate the power absorbed in PR from the relation 1 W1 = 2, (V2—V ̧2 + V22) Watts, where r is the ohmic resistance of QR. 2 If is unknown or liable to be altered by the heating effect r (5) Repeat 2-4 for a different frequency, say 60 per sec., to see whether the Wattmeter "constant" (K) alters, and tabulate your results as follows 1 1 Note. Errors made in measuring the voltages V, V, and V2 or in the graduation of the voltmeter scale will have least effect on the results when V1 = V2. If the formula in 4 is used with the substituted value of ("), this latter may consist of glow lamps, as the resistance may vary with the different mean current strengths. (6) Plot a calibration curve for the Wattmeter tested, having values of deflection d as ordinates and true power W1 as abscissæ. Inference. Prove the formula given in 4 and state any assumption made in obtaining it. What inferences can you draw from the results of your test? and explain why the resistance of the voltmeter V should be large compared with either PQ or QR. (19) Calibration of a High Tension Wattmeter. (By Ohm's Law, using an auxiliary transformer.) Introduction. It is not always possible in actual practice and testing work to avoid the use of a Wattmeter on a high tension circuit, as for instance would be the case in measuring the efficiency of a high tension transformer run off the terminals of a high tension alternator. The Wattmeter in such a case should be a specially arranged one for the following reasons-- (1) Owing to the high pressure in the fine wire moving coil circuit, an extremely high non-inductive resistance, capable of standing the full pressure across its terminals, would otherwise have to be put in series with the fine wire swing coil if an ordinary Wattmeter was employed. (2) Owing to the difficulty in obtaining the above resistance. (3) The risk entailed in handling such an instrument, and of the breakdown of the insulation of the whole arrangement under the high pressure. The best arrangement of a high tension Wattmeter, and which gets over these difficulties, is that shown symbolically in Fig. 19, together with the connections for its calibration. The Wattmeter W consists of an ordinary Siemens electrodynamometer; the mer A m W m cury cups, forming the coil of a small auxiliary transformer T. of T is placed across the high pressure mains (m.m.), hence the moving coil of W passes a current which depends on the E.M.F. of m.m., and at the same time there is no fear from a breakdown of insulation since both coils of W are passing ordinary currents. The actual current which T sends through the swing coil of W may be as small as convenient. Apparatus. High tension Wattmeter W to be calibrated arranged as mentioned above, with its fixed and movable coils separate. Small auxiliary (H.T.) transformer 7; high and low tension electrostatic voltmeters V and v respectively; strictly non-inductive resistance ACB capable of being placed |