An Electron Tube Direct Current Voltmeter of New Design R. L. GARMAN AND M. E. DROZ, New York University, Washington Square College, New York, N. Y.
portioning the current between plate and grid 2. A potential on grid 4 causes an increase in the plate current and a simultaneous reduction of current in grid 2. The “unbalance” of the bridge is consequently almost doubled. The free grid potential of grid 4 is within the range of the voltage of the filament. The customary C-battery is thus eliminated. The proper bias is secured from the filament battery by means of potentiometer R1. B1 is an ordinary 1.5-volt dry cell. For space economy a small type of cell such as the Burgess 4FH is recommended. Bz is a small 45-volt battery.
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LECTRON tube voltmeters employed for electrometric measurements can be classified in two types: batteryoperated and line-operated. Battery-operated voltmeters are characterized by high stability and relative simplicity of design (1, 3). Their principal disadvantage has been the inconvenience and cost of frequent battery replacement due to a relatively high current drain. This disadvantage has prompted the design of line-operated voltmeters ( 2 ) . Battery replacement is eliminated but only at the expense of circuit simplicity, initial cost, and stability with the added disadvantage of operational and constructional complexity.
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Operation as a Titrimeter Operation is extremely simple. With switch SI to the left (Figure 1) the titration cell is connected to the binding posts. To post 1 is connected the electrode
FIGURE 1. DIAGRAM OF CIRCUIT
RI. 1000-ohm uniform volume control I R C 11-108 Ra. 2000-ohm 0.5-watt I R C B T 1/2 Ra. 2600-ohm 0.5-watt I R C B T 1/2 or 5000-qhm semivariable Ra. 50,000:ohm volume oontrol I R C 13-123 with switch cover plate for
Sz IRC-81
B I . Burgess 4FH, 1.5 volts Q. S. P. D. T. switch, Yaxley 732 M . Weston 0-60 microamperes, Model 801 Bz. Burgess W30BP, 45 volts
In spite of these difficulties the trend in late years has been in the design of line-operated equipment (4-8). However, the recent introduction of a new series of 1.5-volt tubes with an exceedingly low filament current makes possible the construction of a much improved battery-operated voltmeter. B y using a circuit wholly new in principle greater stability is obtained and operation is much simplified. This construction, in addition, makes possible a net reduction in the number of component parts for the complete assembly; and the low power consumption (0.3 watt) eliminates the principal objection to battery operation. The circuit is shown in Figure 1. It consists of the pentagrid converter 1A7G in a bridge circuit. The arms of the bridge are (1) the effective cathode to plate resistance, (2) the plate load, Rs, (3) the effective cathode to grid 2 resistance, and (4) the load resistance, Rz. The meter, M , reads the condition of balance of the bridge. R4 controls the sensitivity of the meter and hence of the voltmeter as a whole. Grid 4 controls the electron stream, not in the conventional manner of changing the magnitude of the current but by ap-
FIGURE2. PHOTOGRAPH OF IKSTRUMENT 398
JULY 15, 1939
ANALYTICAL EDITION
399
input voltages of the order of volt. The steady drift after the initial warming-up period is of the order of millivolts per hour and is therefore negligible for all tJitrationmeasurements.
Grid Current The grid current is very low (10-1O ampere) and does not change abruptly from the free grid value as is usually the case with conventional receiving tubes. The grid current is directly proportional to the grid voltage, which indicates that it is due to leakage in the apparatus and to positive ion current within the tube.
Battery Life I n ordinary daily operation a set of batteries should last 6 months; thus the cost of battery replacement is less than 25 cents per month.
Literature Cited (1) Furman, H., IND.ENG.CHEM., Anal. Ed., 2,213 (1930). MILLIVOLTS
INPUT
FIGURE 3. PLOTOF INPUT VOLTSAGAINST METERREADING unit is housed in a cabinet 5.5 X 6 X 5.5 inches-no larger than the familiar internal lamp and scale galvanometer. The total weight is less than 3.5 kg. I n rare instances the titration cell will develop such a high potential that it becomes impossible to secure a zero meter reading by adjusting R1. I n such a case the input leads to the posts should be reversed and R1 adjusted until a maximum meter reading is obtained. As the titration proceeds meter deflections will decrease. If control is not secured in this manner, a battery should be connected in series with the titration cell to reduce the total voltage to a workable value.
Operation as a Voltmeter When the instrument is to be used as a voltmeter and not as a titrimeter, ofiginal adjustment of R1should be made with switch X1 thrown to the right. The meter will read zero for zero external potential. S1 is then thrown to the left to read the potential imposed upon the input binding posts. A plot of input volts against meter readings is shown in Figure 3. Curve A was taken a t full sensitivity and shows that full-scale deflection may be obtained by an input voltage of 100 millivolts. A higher sensitivity may be obtained by doubling the values of Rz and Rs. Curve B shows the operation of the unit a t reduced sensitivity. If only the latter sensitivity is required a 0-500 microammeter may be used instead of the 0-50 microammeter unit indicated in Figure 1. The meter should be connected with the positive terminal to the plate lead, so that increasing the negative voltage a t grid 4 will increase the meter reading. The value of Rz has been chosen to give a zero reading on the bridge meter when the grid is left free. Because of variations in tubes, the specified value of Rz may not produce these same results with all tubes. If the free grid deflection is less than zero the resistor need not be changed, since the grid potential in operation will always be more negative than that a t free grid. If this is not the case the value of Rs must be increased until the desired meter reading is obtained. The proper selection is best made after the tube has been allowed to age about 24 hours.
(2) Garman, R . L., and Droz, M. E., Ibid. 7, 341-2 (1935). (3) Kinney, G. F., and Garman, R . L., J . Chem. Education, 13, 190-2 (1936). (4) Rescorla, A. R . , Carnahan, F. L., and Fenske, M. R., IXD. ENG.CHEM., Anal. Ed., 9,505-8 (1937). (5) Smith, G. F., and Sullivan, V. R. "Electron Beam Spectrometer", Columbus, Ohio, G. Frederick Smith Chemical Co., 1936. (6) Willard, H . H., and Hager, 0. B., IND.ERG.CHEM., Anal. Ed., 8, 144-5 (1936). (7) Working, E. B., Ibid.,10, 397 (1938). (8) Ibid., 10, 434 (1938).
Continuous Supply of Hot Distilled Water GEORGE G. MARVIN Massachusetts Institute of Technology, Cambridge, Mass.
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N CONNECTIOX with the course in qualitative and
Stability
quantitative analysis a t the Massachusetts Institute of Technology, it has become necessary to use large quantities of hot distilled water. The idea of a continuous supply of hot distilled water seemed desirable, and upon investigating the various possibilities the following method was chosen and subsequently installed. The installation consists of a 113.5-liter (30-gallon), gasfired Whitehead automatic water heater containing a Monel tank. Thermostatic controls are an integral part of the unit and the Monel tank is well insulated. All brass pipe, elbows, valves, and faucet were well tinned on the inside previous to installation. The temperature of the water is regulated for 85" C. This temperature is maintained satisfactorily during a period when 60 to 90 liters of hot water are drawn off within about 3 hours. The following test was made to determine the purity of the hot distilled water. With the unit in operation, maintaining a temperature of 85" C., no water was drawn off for a period of about 60 hours. A 10-liter sample was taken, evaporated to a small volume, and analyzed to give 0.35 mg. of nickel per liter of water. No copper was found in this 10-liter sample. The amount of nickel introduced into the distilled water is not considered to be significant except in special work. The unit requires no attention, has proved very satisfactory, and the cost is nominal.
Erratic fluctuations are occasionally visible only when the unit is operating at full sensitivity, and thus correspond to
CONTRIBUTION No. 72 from the Reaearch Laboratory of Inorganio Chemistry, Massachusetts Institute of Technology.
