A direct reading hydrogen-ion meter / by C.G. Pope and F.W. Gowlett.

  • Pope, C. G.
Date:
[1927?]
Catalogue details

Licence: In copyright

Credit: A direct reading hydrogen-ion meter / by C.G. Pope and F.W. Gowlett. Source: Wellcome Collection.

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    any special purpose. The value in ohms of these resistances will be considered later when dealing with the calibration of the instrument. Vernier Rheostat. The controlling rheostat (7) is of the Vernier type, with a total resistance value of about 6 ohms. It is essential that this rheostat shall give very smooth control and be capable of fine adjustment, since it is the only control used in setting the instrument to give standard readings. A suitable rheostat may be obtained from the Presland Electric Supplies Ltd., Hampton on Thames. (The Woodhall Vernier Rheostat.) Shunt Resistance. The shunt resistance (12) and switch (13) are used as a protective device for the meter. This resistance has a value of about 2 ohms, and the circuit is kept closed until the button, seen immediately below the curve control in Fig. 1, is depressed. Rotary Switch. The rotary switch (16) is a double pole three way switch which connects the grid of the valve to the standard cell or to the desired electrode terminals (16a) and at the same time connects the correct resistances in the meter circuit (16b), viz. resistance (10) when the standard cell is in circuit, resistance (9) for the hydrogen electrode system and (11) for the quinhydrone system. The switch shown in Fig. 2 was originally made for a different circuit and appears therefore, to be more complicated than is necessary. Of the studs shown in the photograph only the extreme end and centre ones on each side form part of the circuit. Method of Calibration The filament portion of the circuit, which resembles an ordinary potentiometer circuit, will be dealt with first. The anode battery may be taken as having a potential of from 11*4 volts to 13*2 volts, according to its condition of charge. Under these circumstances a resistance of a value varying from 28-4 to 33 ohms will pass a current of 0*4 ampere, which is required for the valve. Since the valve filament takes 0-4 ampere at 2*0 volts its resistance is 5 ohms. It will be seen that resistances (4), (5), (6) and (7) should have a value varying between 23*5 and 28 ohms, the variations being obtained by means of the rheostat (7). If the grid bias resistance (6) has a value of 0-75 ohm there will be a difference of potential of about 0*3 volt between its ends, which is sufficient to ensure that the grid is always at a slightly negative potential with respect to the filament. The value for the combined resistance (4) and (5) is about 22 ohms, of which 19 ohms is included in (4) and 3 ohms in (5). Thus the potential difference between the ends of resistance (5) is about 1-2 volts. The circuit was arranged in this manner in order to keep the combined resistance of (8) and either (9), (10) or (n) high (i.e. 1000 ohms approxi¬ mately) compared with the resistance of the meter. The ratio obtained is approximately 1000 ohms to 10. The General Principle of Calibration of the Meter The object of calibrating is to obtain the condition where the change in e.m.f. for a difference of one pVi unit, when applied to the grid of the valve, causes a change in the anode current so that a deflection of exactly 10 divisions is produced on the meter. The change in anode current for a given grid potential variation may be greater or less than the required amount. In this case it is necessary to alter the slope of the anode current curve by varying the filament temperature. Increasing the filament temperature by reducing the resistance at (7) makes the anode curve steeper and vice versa. The apparatus is calibrated by applying known potentials by means of a potentiometer. A variable resistance box (reading to say 10,000 ohms) is used in place of resistances (9), (10) and (11), these resistances afterwards being wound to the value shown by the variable
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