Improved Kundt's Tube MYRON B. REYNOLDS Brown University, Providence, Rhode Island
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EARLY everyone who has worked in an undergraduate physical chemistry laboratory is familiar with Kundt's tube as i t is used to measure the velocity of sound in gases. In its conventional form, Kundt's tube is excited by strok'mg a rod attached to a dia&agm within the tube, and when the movable piston has been adjusted to resonance, the distance between nodes is determined by means of lycopodium powder sprinkled inside the tube. This is a rather makeshift method, and the instructors in the laboratory a t Brown University improved upon it by exciting the tube with a loudspeaker driven by a General Radio audio oscillator. When the writer first saw this improvement a few years ago, it occurred to him that, since the electrical impedance of a loudspeaker is a function of the acoustic impedance to which it is coupled, there should be a noticeable change in the impedance offered by the speaker when the piston Input. 200". in the tube is adjusted to the resonance position. A FIGURE1, simple vacuum tube voltmeter using a type 6C5 tube was connected across the primary of the transformer couplmg the voice coil of the speaker to the oscillator. plate-cathode resistance becomes a function of the Some change in the plate current of the tube could alternating current input to the grid. If this condition be observed when the piston was adjusted to resonance, is met, the two plate-cathode resistances will change but the sensitivity was not all that could be desired. with changes in the alternatingcurrent inputs to the two This device was used for a year or more in spite of its grids, but so long as the alternating current inputs to the disadvantages, since it was better than the lycopodium twogrids change in the same ratio, the two plate-cathode powder method. resistances should maintain the same ratio so that the Finally, a t the suggestion of Prof. J. P. Howe, the bridge balance is not changed. From Figure 1 it may more sensitive device described was constructed. be seen that the alternating current input from the Time has shown i t to operate in a very satisfactory coupling transformer Ta is divided between the two manner. Except for the oscillator, galvanometer, grids in a ratio determined by the relative impedances and galvanometer shunt (which are likely to be avail- of the primaries of the two transformers TIand T2. able in any physical chemistry laboratory), the entire These impedances are determined by the value of the apparatus can be constructed from standard radio resistance Rs and the impedance of the voice coil of parts. Any undergraduate with some knowledge of the speaker S. At any given frequency, if the acoustic radio circuits and construction can build the apparatus load on the speaker remains constant, the relative in a few hours. impedances offered by the two transformers TIand TI It will be seen from Figure 1 that the circuit used should be independent of the alternating current input is the old familiar bridge, with the plate-cathode from Tg,SO that the inputs to the two grids will mainresistances of the vacnn& tube forming-two arms of tain the same ratio and the bridge balance he inthe bridge. The tube must be biased nearly to the dependent of the alternating current input. Since in cut-off point in order that the effective direct current actual practice it is impossible to obtain a tube with 121
I
I -1
122 the characteristics of both halves identical, this condition cannot be completely achieved. However, the variations in the output of a good audio oscillator are sufficiently small that no diiculty should be encountered from this source. Now suppose the load on the speaker be changed, in this case by adjusting the piston in the Kundt's tube to the resonance position. This merely changes the impedance offered by TIwithout changing that offered by T3. The ratio of the two grid inputs changes, changing the ratio of the two plate-cathode resistances and unbalancing the bridge, as indicated by the galvanometer G. The construction of the apparatus is fairly simple. The electrical parts should be so laid out as to give short, direct leads. There is no reason why two single triodes could not be substituted for the 6N7 tube, and the circuit constants be changed to suit the tube (or tubes) used. The plate supply for this apparatus may be either "B" batteries or a rectifier unit. In the original apparatus a 6.3-volt transformer was used to supply the power for the cathode heater of the 6N7 (not shown in the diagram), although a battery could be used. The constructor may find it desirable to experiment a bit to find what parts may be substituted for the ones described. The physical details of the actual Kundt's tube are left to the constructor. In the original a 50-millimeter glass tube was used. The speaker was attached to a metal flange which was fastened to the glass tube with deKhotinsky cement. The loud speaker end of the tube was enclosed in a wooden box about a foot square and filled with cotton. This should be clear from Figure 2. This box is not absolutely necessary, but it saves the ears of those workmg in the vicinity of the apparatus. The actual operation is very simple. Before the apparatus is turned on, the galvanometer shunting resistance should be reduced to a few ohms. After the oscillator, bridge, and power supply are turned on and warmed up, the galvanometer is brought as near to zero deflection as possible by adjusting &. The galvanometer shunting resistance may now be increased and Rg adjusted simultaneously to keep the galvanometer deflection as small as possible. The final fine adjustment of the galvanometer deflection to zero is made by adjusting RI. The bridge is now
ready for use, and when the piston of the Kundt's tube is adjusted to resonance, the galvanometer deflection will indicate the resonance position. If the galvanometer goes off scale a t the resonance position, the sensitivity may be decreased by decreasing the shunting resistance Re.
The writer wishes to thank Professor J. P. G ~H : for his helpful suggestions and cooperation during the construction of the original apparatus.
RI
= 1000-ohm wire-wound potentiometer, Yaxley or equiva-
Rn
=
Ra &
= = = = = = = =
R6 Re R, R8 C C,
T = S = TI = T2 =
TZ
=
G
=
lent 10,000-ohm l-watt carbon-resistor 10,000-ohm l-watt carbon-resistor 0.5-megohm l-watt carbon-resistor 0.5-megohm l-watt carbon-resistor Four-dial resistance box. 0 to 9999 ohms 10,000-ohm 1-watt carbon-resistor 10-ohm wire-wound rheostat, Yaaley or equivalent 0.1 to 1.0 pf. paper condenser of good quality 0.5 af. paper condenser. may he omitted 6N7 tube Oxford 2ZMP permanent magnet speaker, 2 inch Thordarson T-13S42 universal output transformer Tbordarson T-l3S42 universal output transformer General Radio type 578-A or Thordarson T-61S26 Galvanometer, in original apparatus characteristics mere: Resistance, 1047 ohms Critical damping resistance, 7200 ohms Period, 2.25 seconds Sensitivity. 0.0272@mp./mm. Any galvanometer of similar characteristicsmight be used
