Patent GB-583238-A (GB)
Title: Improvements in or relating to arrangements for measuring electric power
IPC: G01R21/00
Filed: 1944-06-16
Abstract: 583,238. Wave guides. STANDARD TELEPHONES & CABLES, Ltd., and MONTGOMERY, W. A. June 16, 1944, No. 11549. [Class 40 (v)] [Also in Group XXXVI] An arrangement for measuring power, particularly at high-frequencies, comprises a Wheatstone bridge wherein one arm consists of a temperature - dependent resistance, the opposite arm consists of a fixed resistance and the other two arms consist of simultaneously adjustable resistances, means being provided for subjecting the temperature dependent resistance to the waves or currents whose power is to be measured and for measuring the power dissipated in the fixed resistance arm which is proportional to the power dissipated in the temperature-dependent resistance when the bridge is balanced. Fig. 1 shows a bridge, the opposite arms 1, 2 of which comprise variable resistances, the controls of which are linked so that their resistances ratio is a constant k. The third arm consists of a bolometer lamp 3 or other temperature-dependent resistance having a +ve or - ve resistance-temperature coefficient and the fourth arm consists of a fixed resistance 4 in series with a current measuring instrument 5. It is shown in the Specification that when the bridge is balanced the power dissipated in the arm 3 is equal to the power dissipated in the arm 4 divided by the constant k. This value is proportional to the square of the current in the arm 4 so that the current meter 5 in this arm may be calibrated to read directly the power dissipated in the temperature-dependent resistance 3. In operation, the bridge is balanced by means of the coupled resistances 1, 2 and galvanometer 6, the current in arm 4 being adjusted to a suitable value by the variable resistance 12 or any other. suitable means in the bridge input circuit from source 9 which may be A.C. or D.C. The bolometer lamp 3 is then subjected to the waves to be measured, filter arrangements being provided to prevent this energy reaching the other bridge arms. The resistance of arm 3 is thereby altered to upset the bridge balance which is then adjusted by the element 12 until again balanced. The difference of the two readings of meter 5 is then a measure of the power of the waves applied to the arm 3. For fine adjustments, the resistances 1, 2 and 12 may each comprise a fixed and a variable portion. Fig. 2 shows the bolometer lamp 3 connected to measure the power in a coaxial line or wave guide. The lamp 3,is connected between the central conductor 14 and a plate 16 insulated by a mica or other disc 17 from the flanged end 15 of the outer conductor 13. The points 7, 11 of the bridge circuit of Fig. 1 are connected respectively to the flange 15 and to the plate 11. A side tube 18 containing a central conductor 19 and of length equal to a quarter of the wave-length of the H.F. waves arriving from the left-hand end of the line 13 is shortcircuited at its outer end to provide an electrical connection between the point 7 and the bolometer lamp 3. This side tube at the same time presents an infinite impedance to the highfrequency energy, substantially all of which is therefore absorbed by the bolometer lamp 3 and measured as indicated above. The elements 15, 17, 16 form a byepass condenser for the high-frequency waves so that the plate 16 substantially closes the 'end of the guide to these waves without short-circuiting the points 7, 11. The side tube may be provided with a tuning piston. For measuring the power of a high-frequency wave coming along a wave guide 20, Fig. 3, the bolometer lamp 3 is located within the guide and connected between the central conductor 26, 27 of side tubes 22, 23 insulated from the guide 20 by mica sheet 24, 25. The outer ends of the conductors 26, 27 are connected to the bridge points 7, 11 of Fig. 1 and tuning pistons 21, 28, 29 are adjusted so that substantially the whole wave energy is absorbed in the resistance 3. The insulating sheets 24, 25 form with the side tube flanges and the guide 20 condensers for the passage of the high-frequency energy to the side tubes. For measuring the power of a low-frequency wave, Fig. 4 (not shown), filtering arrangements which may be tuned to the L.F. are provided in the wave input circuit to the bridge points 7, 11 in the bridge arm 7, 10 and in the diagonal 7, 8 so that the L.F. wave is confined to the bridge arm 7, 11 and the local bridge arm currents are .excluded from the circuit of the L.F. wave being measured. The source 9 for the bridge, if alternating must be of a frequency sufficiently removed from the frequency of the wave being measured to admit of satisfactory filtering and in such cases a telephone may be used in lieu of the galvanometer 6. The frequency of source 9 is not necessarily below that of the waves being measured.