A FLEXIBLE STUDENT CONDUCTIVITY BRIDGE ASSEMBLY W. I?. LUDER AND A. A. VERNON Northeastern University, Boston, Massachusetts I.
INTRODUCTION
S
EVERAL conductivity bridges have been described recently,' which offer considerable improvement over the performance of the slidewirehummer bridge for student use. However, these setups have two disadvantages in such use. First, the
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TO OSCILLATOR
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PHONES
appearance ol the apparatus is such that it is not obvi011sto the student that he is workinrr with a Wheatstone bridee.% Second, each set is desimed as a separate unit 1 For example. Cf., EVANS, THISJOURNAL,15, 389 (1938);
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which makes i t difficult to use a number of them &ciently in any but the smallest classes. The bridge here described has been designed (1) to look like a Wheatstone bridge and (2) to operate from a power supply common to i t and several other similar bridges. Each student (or group of students) performing an experiment has his own set of measuring apparatus, but each set is connected by a plug to the same power supply. The power supply may be used with as
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H A T ~ E LAND D STRICRLER, ~ b d . 15,381 , (1938).
many as ten or twelve sets. Each individual set consists of an oscillator, a resistance box, headphones, two variable condensers, and a control panel. Upon this control panel the four arms of-the Wheatstone bridge are laid out in such a manner that the bridge itself serves as the only wiring diagram the student need consult. The sensitivity of the bridge is considerably better than 0.1 per cent. (When measuring resistances above 1000 ohms, it is better than 0.01 per cent.) The accuracy is also better than 0.1 per cent., provided a good resistance box is used. Such accuracy is greater than that usually needed for student use. No sound whatever can be beard in the phones when the bridge is balanced. The cost of a number of outfits is less than the cost of the same number of any other type of which we know. This is true even if the-cost of h&ng them assembled by a radio repair man must be included.
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THE BRIDGE NETWORK
The diagram of the bridge is given in Figure 1 and its photograph is given in Figure 4. The plywood board has dimensions of twelve by fifteen inches and is varnished with two coats of bakelite spar varnish. The ratio-arms are General Radio type 500 plug-in resistors. One of 100 ohms, one of 1000 ohms, and two of 10,000 ohms resistance will give sufficient variation in the ratio of the arms for most purposes. For many measurements the bridge can be used as an equal ratioarm bridge and the resistance of the cell will be given directly by the reading of the resistance box (R in Figure 1). The two 10,000-ohm resistors should then be used as ratio-arms. If one of the 10,000-ohm resistors is replaced by the 100- or 1000-ohm resistor, then the reading on R must be multiplied or divided by one hundred or ten as the case may be. If an ll.ll0-ohm three-dial box is used for R, the range of the bridge is 1,111,000 ohms. A four-dial 11,111-ohm box will, of course, make for greater convenience in reading with less changing of the ratio arms. In tests, the same resistance measured with different ratios of the plug-in resistors gave the same resistance reading within less than 0.1 per cent. With the oscillator output transformer used, the sensitivity of the bridge falls off rapidly if the impedance between the oscillator terminals becomes less than about 500 ohms. The cell constant of the measuring cell should be so
sections may be connected together in parallel as demanded by the characteristics of the output transformer in the case of C,, or by the capacity of the cell in the case of C,. The Wagner ground must be rebalanced for every resistance reading. To balance the bridge proper, C, and R are adjusted together until no sound
is heard in the phones. Binding post G is connected to ground through a near-by water pipe. The oscillator ground lead may be brought toG or directly to a water pipe. .. 111.
chosen as to make the resistances of the solutions come in the range of 20C-100,000 ohms. In measuring resistances approaching the lower limit it is advisable to use a ratio of plug-in resistors such that R has as high a value as possible. In any case either one or the other of the 10,000ohm resistors should always be plugged in. To secure a silent balance requires a capacity balance and a proper grounding system. The Wagner ground has been described several times.$ R, (Figure 1) may have a value of 50,000 ohms. C, and C, may be three-gang radio tuning condensers. Two or three
THE OSCILLATOR
The oscillator is shown in Figure 2. The circuit was taken from an R.C.A. bulletin3 and modified to secure a higher output voltage. The original form of the oscillator is in use in this laboratory with a precision bridge and is, everything considered, probably the best available for the purpose. Its distortion-free output of 0.5 volt is, however, too low for use with a student-type bridge when no amplifier is used between bridge and phones. By changing the output leads from the grid coil to the proper plate resistor the distortion-free output voltage becomes about 10 volts. The oscillator is inexpensive and easy to construct. L is a 1500-turn honeycomb coil. Cis a 0.1 MFD. and two 0.01 MFD. paper condensers in parallel. T is an inexpensive 2 :1 or 3 :1 audio transformer. The parts are mounted in an eight-inch cubical shield with hinged The 500,000 ohm regeneration control is mounted on the front panel. The four-prong socket for the 3 "Technical Service Bulletin TS-2," R.C.A. Manufacturing Company, Inc., Harrison, New Jersey, p. 97. 'All the parts are easily obtained from amateur radio shops or catalogs except the 1500-turn honeycomb coil. I t may be obtained from the Radio Shack. I67 Washington Street, Boston. Massachusetts.
power supply plug is placed on the rear panel. The output jack, which must be insulated from the case, is placed on one side. When L and C have the values given, the frequency of the oscillator output is approximately 1100 cycles. When the regeneration control is properly adjusted the voltage will be about 10 volts practically distortion-free as shown by a cathode-ray oscilloscope. The control is adjusted b$ turning the knob until the 1100-cycle note is heard, then retarding it until oscillation stops, then advancing it slowly until oscillation just begins again. N.
THE COMMON POWER S U P P L Y
The power unit which supplies filament and plate voltages to all the oscillators is conventional in design. ~achoscillatorrequires 2.0 amperes a t 2.5 volts for the filament and 3.5 milliamperes a t 250 volts for the plate of The transformer in the Dower s u ~ ~must lv - - the ~ - 53. - ~ be chosen according to the maximum number of oscillators to be used. In case as many as eight or ten are needed it is probably best to use an extra 2.5-volt filament transformer for the filaments of half of them. Since the filament current is so high, each pair of filament leads to the oscillators should be taken off as close to the power supply plug as possible. Heavy lamp cord should be used. The whole assembly should be grouped about the thermostat in which the conductance cells are placed in such a manner as to make the power leads to the oscillators as short as Dossible. Each oscillator should be four or five feet away from the other parts of each set. ~
resistors for the ratio arms. Thus the measured resistance can be read directly or in multiples of ten. (2) The bridge network is designed to look like a Wheatstone bridge. (3) The bridge balance is sharp to better than 0.1 per cent. No sound can be heard when the bridge is balanced. Depending on the resistance box used, the accuracy of measurement may be better than 0.1 per cent.
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SUMMARY
(1) The bridge assembly described is flexible. Several measuring sets may be used with the same power supply. The bridge networks are fitted with plug in
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POWERSUPPLY A N D OSCII.I.ATOR
(4) An adaptation of a new low-distortion oscillator provides a simple and inexpensive - oscillator for each measuring set. (5) The assembly is inexpensiue arid easy to construct. The cost of each set exclusive of the resistance box is approximately twenty-five dollars. The cost of the power supply to all the sets is about nine dollars.
