Fig. 2,616.—Diagram of connections of Duddell oscillograph to high pressure circuit. The modification necessary for high pressure circuit only applies to the vibrator which gives the pressure wave and consists in adding two more resistances, R4 and R5. Referring to [fig. 2,617], it will be seen that in case fuse f2 blows, or the vibrator be accidentally broken, the full supply voltage is immediately thrown on the instrument itself. This is not permissible in high voltage work and therefore the resistance R5 is introduced as a permanent shunt to the oscillograph vibrator. The resistance R4 is an exact duplicate of R2 being a 21 ohm plug resistance box for adjusting the sensitivity of the vibrator to an even figure. In practice R5 is usually a part of R1, and in most of the high voltage resistances, two taps are brought out near one end to serve as R5. One of these taps is usually 50 ohms distant from the end terminal and the other only 5 ohms from the end. The use of these taps is as follows: The large resistance consisting of R1 + R5 is so chosen with respect to the voltage of the circuit under investigation that the current through R1 is about .1 ampere. It should never be more than this continuously. Then R4 is connected to the 50 ohm tap, and since the resistance of the oscillograph vibrator circuit is variable from about 5 to 26 ohms by means of R4, the current can be controlled through the oscillograph from about .066 to .091 of an ampere, enabling an open wave form to a convenient scale to be obtained. If it now be desired to record large rises of pressure, such as may occur in cases of resonance, the height of the wave must be reduced in order to keep these rises on the plate. This is accomplished by disconnecting R4 from the 50 ohm tap and connecting it to the 5 ohm tap, when the current through the vibrator will be from .05 to .016 of an ampere according to whether the resistance R4 is in or out of circuit. When, instead of using the falling plate, the cinematograph camera is being used, it becomes necessary always to work on the 5 ohm tap since the width of the film is much less than that of the plate, and the current must therefore be less. In experiments where sudden rises of voltage are expected it is often advisable to keep R1 as great as possible. That end of the resistance R1 referred to as R5 in the diagram should be securely connected to the supply main and no switch or fuse used. A switch may, if desired, be used in series with R1, provided it be inserted at the point where R1 joins the supply main remote from R5. It will be seen that fuses f1 and f2 are shown. Provided that the connections are always made in accordance with the diagram, and the vibrators are always shunted by R5 or R3 respectively, there is not much objection to the use of these fuses, but on general principles it is wise to avoid fuses in high tension work and accordingly with each permanent magnet oscillograph, dummy fuses are supplied, which can be inserted in place of the ordinary fuses when desired. The remark previously made about keeping both vibrators and the frame of the instrument at approximately the same pressure applies with additional emphasis in high pressure work.
Ans. They simply control the direction of a beam of light in a horizontal plane in such a manner that its deflection from a zero position depends on the current passing through the instrument, and it is therefore evident that the oscillograph is not complete without means of producing a time scale.
Fig. 2,617.—Diagram of connections of Duddell oscillograph to low pressure circuit, R1 is a high non-inductive resistance connected across the mains in series with one of the vibrators. S2 is a switch, and f2, the fuse (on the oscillograph in this circuit). The resistance of R1 in ohms should be rather more than ten times the voltage of the circuit, so that a current of a little less than .1 of an ampere will pass through it. The vibrator will then give the curve of the circuit on an open scale. (For the projection oscillograph, the resistance R1 should be only twice the supply voltage, since .5 of an ampere is required to give full scale deflection on a large screen.) To obtain the current wave form, the shunt R3 is connected in series with the circuit under investigation and the second vibrator is connected across this shunt. Here also f1 is a fuse, S1 a switch, and R2 an adjustable resistance box. The switch S1 is however unnecessary if the plug resistance box supplied for R2 be used, since an infinity plug is included in this box. The shunt R3 should have a drop of about 1 volt across it in order to give a suitable working current through the vibrator. The resistance R2 is not absolutely essential, but it is a great convenience in adjusting the current through the vibrator. It is a plug resistance box, the smallest coil being .04 of an ohm and the total 21 ohms. Being designed to carry .5 ampere continuously it can be used with any other type of Duddell oscillograph, and by its use the sensitiveness of the vibrator can be adjusted so that a round number of amperes in the shunt gives 1 mm. deflection. This adjustment is best made with direct current. It should be noted in connecting the oscillograph in circuit, that the two vibrators should be so connected to the circuit that it is impossible that a higher pressure difference than 50 volts should exist between one vibrator and the other, or between either vibrator and the frame. To ensure attention to this important point, a brass strap is provided which connects the two vibrators together and to the frame of the instrument. This does not mean that this point must necessarily be earthed since the frame of the instrument is insulated from the earth. It is advisable, however, to earth it when possible.
