> > > >

1947-48 Theatre Catalog, 6th Edition, Page 364 (350)

1947-48 Theatre Catalog, 6th Edition
1947-48 Theatre Catalog
1947-48 Theatre Catalog, 6th Edition, Page 364
Page 364

1947-48 Theatre Catalog, 6th Edition, Page 364

Furthermore, the loops are disposed that the median point or center 25 of loop 20 coincides with the median point or common center about which loops 21 to 24 are arranged.

Figures Four and Five

Figures 4 and 5 show further arrangements of transmitting conductors in the form of loops, arranged in series and presenting the same features as those disclosed in Figure 3. By thus having the loops arranged about a common median point or center, the neutralization of the effects at a distance of the loops is obtained to a further degree than is possible if a series of loops are disposed for example as shown at 7 to 10 in Figure 1 in which the center of the field formed by the loops 7 and 8 does not coincide with the center of the field formed by the loops 9 and 10. As shOWn in Figure 4, the conductor forms four loops 4a, 4b, 4c, and 4d -of which the intermediate loops 4b and 4c have current fiowing in the same direction. Thus the field produced between them will be weak and they should be placed close to each other. This arrangement is particularly advantageous where reception is more particularly required at the outer part of an enclosed area as in the case of sports arenas, for example, a football ground, where the central part of the enclosed space is occupied by the


Figure Six

As shOWn in Figure 6 a receiving apparatus suitable for deaf aid purposes may comprise a thermionic amplifier having three thermionic valves 26, 27, and 28. In the input circuit of the grid of the thermionic valve 26 is a pick-up coil 29 which is screened against electrostatic reaction and interference by metallic discs 30 and 31. The screening discs, unless of low conductivity, should be perforated or slotted to reduce induced currents. A potentiometer 32 is used for volume control. It is generally desirable to apply slight bias to the grid of the valve 26 to which the pick-up coil is connected, as otherwise the grid current may cause the impedance of the pick-up circuit to drop sufliciently to alter adversely the frequency response curve of the amplifier. Bias for the first and second stages of amplification is obtained by the voltage drop across an electric fiash lamp 34 arranged in the cathode circuit and bias for the output valve 28 is obtained by superimposing on the voltage drop across the electric fiash lamp 34, a voltage drop across a

resistance 35 which is shunted by a condenser 36. A headphone 37 is arranged in the anode circuit of the thermionic valve 28. With a receiving apparatus as shown, it has been found that decoupling of the high-tension current for the thermionic valve 27 is unnecessary. Instead of applying bias as set forth above, the grid of the valve 26 may be made self-biasing by incorporating in the grid circuit a condenser 62 shunted by a high resistance 63, in which case the electric lamp 34 may in some circumstances be dispensed with.

Figures Seven and Fight 4'1

Figure 7 shows diagrammatically an arrangement of deaf aid appliance including a capacity microphone adapted for use as a receiving apparatus in connection with a transmitting apparatus as shown in any of the Figures 1 to 5. By means of a switch 38, a microphone 39 or a suitable screened pick-up coil 40 can be connected to the grid of a valve 37 as desired. The pick-up coil 40 is shunted by a resistance 41, whilst the said coil and resistance are in series with a condenser 42. With this arrangement, the receiving apparatus can be employed for receiving the signals transmitted through a transmitting conductor 4 by connecting the pick-up coil 40 to the grid of the thermionic valve 37, or it may be employed as a normal deaf aid appliance by connecting the microphone to the grid. It is preferable that the condenser 42 shall have a capacity similar to that of the microphone 39 so that when the pick-up coil is connected to the grid, the grid load may be substantially capacitative over all or a substantial part of the frequency range to be received, a condition that is desirable, for example, when negative reaction is applied to the grid for volume or tone control.

Figure 8 shows a further arrangement of receiving apparatus. A switch 44 is adapted to connect the primary winding of a step-up transformer 45 either to a carbon microphone 46 or to a pick-up coil 47 as desired. The secondary winding of the transformer 45 is connected to the grid of the thermionic valve 37 and also to the cathode of the valve through a condenser 48 that is shunted by a resistance 49 whereby bias is acquired by the grid of the valve 37. On account of low impedance of the coil in this case, the screening is generally unnecessary.

Figure Nine

Figure 9 shows a modification of the receiving apparatus in which the microphone and the coil can act simultaneously in variable degrees controlled by a potentiometer 57 on the grid of a thermionic valve 51 respectively through resistances 52 and 53 and through resistances 55 and 56. The resistances 52 and 55 should be of such values that the loads of the microphone 50 and of the pick-up coil 54 are substantially resistive over the range of frequencies to be received whilst resistances 53 and 56 should be of a like value and that of potentiometer 57 somewhat higher. In order to obtain volume and tone control one or more of the resistances 52, 53, 55, and 56 may be made variable or shunted by a condenser to modify the tone. Thus, for example, condensers 58 and 58' may be arranged to shunt respectively the resistances 53 and 56.

Other Details

In order to obtain the most efficient reception, it is usually preferable that the pick-up coil shall be parallel to the plane or planes of adjacent loops forming the transmitting conductor, but, in some cases, as when the receiving apparatus is situated immediately above or below the conductor, reception is very poor or negligible until the pick-up coil is tilted out of parallelism to the plane or planes of the said loops of the transmitting conductor.

Where wire of a light gauge is used for the transmitting conductor, the ohmic resistance of the Wire Will be large compared with its inductance over the range or part of the range of frequencies to be received and the current flowing in the coil will be substantially independent of frequency. 'Where the resistance of the transmitting conductor is insufficient in itself, a fixed or variable resistance may be connected in series to render the load resistive over the whole frequency range. Tone control may be effected by varying such resistance. The use of such a resistive load is particularly applicable where no separate output stage is employed but the transmitting conductor is connected in parallel with a resistive load such as loudspeakers. A receiving apparatus as hereinafter described with amplification independent of frequency must be used in conjunction with such a transmitter if a uniform frequency response is required. On the other hand when the transmitting conductor consists of a heavy cable or copper foil its inductive impedance will usually exceed the ohmic resistance thereon over the material range of frequencies. In this case for frequencies at which the inductive impedance of the transmitting conductor is greater than that of the impedance of the output circuit to which it is coupled, the current produced in the transmitting conductor by a given output voltage falls off with rising frequency and tends to become inversely proportional to frequency when the inductance is large compared with the impedance of the said output circuit and the receiving apparatus and pick-up coil may be arranged to have an amplification proportional to frequency. In this latter case, where as uniform frequency resonse as possible is required, the output of the amplifier should be adjusted according

1947-48 Theatre Catalog, 6th Edition, Page 364