60-Cycle Source for Operation of Laboratory Timers. Kendell J. Biermann and Neil1 Weber, University of Manitoba, Kinnipeg, Canada. and other investigations substantial errors can Imotor-driven result from the measurement of time intervals by synchronous timers operated from the power line ( I ) . It has N CALORIMETRIC
been necessary, therefore, to employ special power sources for the operation of such timers. The circuit shown in Figure 1 has been used to drive a 5-watt synchronous motor, of the type used in Standard Electric Co. timers. Time intervals measured with this apparatus agreed to within 0.001% with the intervals between time signals broadcast by the National Bureau of Standards radio station WWV. A 90-kc. frequency is generated by the crystal-controlled 6SJ7 oscillator ( 3 ) . Condenser C (two-section, variable air condenser, 365 mmf. per section) permits slight frequency corrections, thus allowing the use of an inexpensive crystal such as the James Knight Type H-18, provided that the crystal is mounted in such a manner that i t does not become heated and the circuit is allowed some warm-up time before use. Isolation of the oscillator is accomplished with the 6557 buffer amplifier. Frequency division to 60 cycles takes place through a series of five multivibrators, each synchronized to an exact submultiple of the oscillator frequency by injection of part of the output of the previous stage through an R-C differentiating network, the frequency divisions employed being successively 3, 5 , 5 , 5 , and 4. While it would be possible to lessen the number of stages by use of larger frequency divisions per stage, conservative design ( 2 ) requires that maximum division per stage be less than ten. The output of the last multivibrator is amplified through a conventional phase inverter and push-pull amplifier; the potentiometer in the phase inverter input is adjusted to deliver 115 volts at the transformer secondary when the load is connected. Sufficient rounding of the corners of the square wave input takes
Figure 1.
place in the amplifier and tiansfoiniei 5 0 that the output is approximately sinusoidal. In order to eliminate any effects of line voltage fluctuation, the voltage to the oscillator and frequencj divider stages is supplied through a conventional electronic voltage regulator circuit. I t is virtually impossible to anticipate the free running frequency of a multivibrator. It is therefore necessary to include sufficient variability in each multivibrator stage to permit adjustment of its free running frequency sufficiently close to the desired subharmonic of the crystal frequpncy to allow synchroniaation to this subharmonic to take place. To this end a variable resistor is placed in each of the frequency divider stagpa in such position that the time constant, and hence the free running frequency, can be altered over a large enough range to cover the normal deviation of components from their nominal values. These variable resistors can be set by observing with an oscilloscope the wave form on that grid of the multivibrator tube to which the synchronizing pulses are applied. As shown in Figure 2, the wave form consists of a superimposition of the synchronizing pulses on the square wave generated by the multivibrator; the number of pulses on each square wave cycle is the frequency division of that stage. The variable resistors are adjusted, starting a t the high frequency end, to the center of the region in which the desired frequency reduction takes place. The condenser C can then be adjusted, if necessary, by comparison of timer intervals a i t h WWV time signals. Comparison is conveniently made by engaging the timer clutch on an hour signal and disengaging it on a subsequent hour signal. If the interval between the two signals is about 10 hours, the error in manual synchronization of the clutch and time signal will be negligibly small. Although the 60-cycle generator used in this laboratory has shown no detectable change in calibration over a period of 6 months, the comparison with WWV intervals should be repeated periodically. The components used in the frequency divider stage5 ihould be
Circuit Diagram of 60-Cycle Power Source
1284
V O L U M E 25, NO. 8, A U G U S T 1 9 5 3
1285
I
I Figure 2.
Oscilloscope I'attern for 5 to 1 Frequency vision
within 5% of nominal and due caution should be observed to protect components from exeefisive heat insoldering. A heavy brass clamp applied to the lead wire between the component and the soldering terminal is conveniont for the latter precaution. ACKNOWLEDGMENT
This circuit was developed a6 part of a project aided financially by the Research Council of Canads. LITERATURE CITE1 (1) Craig, R. 8.. Satterthuwsite, C. B.. and CHEM., 20,555(1948). (2) Cruft Laboratory Staff, "Electronic CirLrlvr XXIV, New York, McGraw-Hill Book Co., 1947. (3) Tibbetts, D. R., E'lecfonies, 14.35 (October 1941).
-.."
~
ALL.
-hap.
Plastic Stirrer Blades. Verne11 R. Shellman and Barney J. Magerlein, Research Laboratories, The Upjohn Co., Kalamazoo, Mich.
chemieah, being attacked only by molten alkali metals. It is unaffected by temperature up to about 225' C. The stirrer blade shown on the right is patterned after a glass stirrer blade commercially available (Trubore stirrer blade, Ace Glass, Inc., Vineland, N. J.). The two stirrers shown on the left are reminiscent of the wire stirrer described by Hershberg [TND.ENG.CHEM.,ANAL.ED., 8, 313 (193611. These stirrer blade8 are cut from '/,sinch or '/&ch sheets of Teflon. They are readily fashioned using a Isboratoiy cork borer and a sharp knife. The segments of one type of stirrer are gently twisted, and will maintain their shape in most laboratory reactions. The two blades shown on the left are snapped onto the stirrer Ehaft loou throueh a cut in the center seement of the blade, This arrangement, which has withstood much wear, permits the interchangeable use of a variety of sizes of blades on the same shaft. The Teflon stirrer blades are superior to their glass or wire counterparts, being readily con8tructed and virtually indestivcb ible. These blades are easily cleaned, as few things adhere t o i do the the smooth plastic. They do not scratch I glass or wire blades. I
~
Precise Speedy Pipet. C. H. Whitnah, Ktr~w,rmmt. College, Manhattan, Kan.
s
EYERAL workers have studied the speed, accuracy, and pre-
cimon of pipets ( 1 , 3, 4). The pipet described was built ( d ) to measure pyridine sulfate dihromide reagent for iodine numbers by a modified Rosenmund and Kuhnhenn method ( 6 ) . It wm desired to measure two blanks and four duplicate determinations so quickly and precisely that the density and concentration of the reagent could be considered constant during these ten measurements. The pipet (Figure 1) may be considered to be a modified Koch buret. "
Closing the s u u d v bulb with a calcium chloride tube urevented
wo types of laboratory stirrer blades made of Teflon (E. I.
Tdu Pant de Nemours & Co. Inc., polymers of tetrafluoroethyl
ene) have been satisfactorily used in this laboratory for several months. This plastic is noted for its outstanding resistance t o
A mirror'was~~laced~back of this tube, and scratches on this
and could be partly filled with Gater from a leveling &lb with stopcock. A stopcock, near the top of the reservoir, could be ._._. 1 . 2 : . . -
..... ~~. :~~ 1 1 ~
~
adjusted, increased pressure would stop flow of reagent into the measuring tube just above the upper scratch, and decreased pressure would stop flow out of the tip just below the lower scratch. These pressure changes also hastened the initial flow rate of cach filling and emptying, and slowed down the find rates. The slow h a 1 rates greatly sided in adjusting the level of the meniscus and its image exactly to the scratches. The volume of air in the resrrvoir must be increased as the level of reagent in the supply bulb is lowered. It was adjusted so that the measuring tube would just fill and just empty when the supply bulb was full. The lowest reagent level, after each of 8everd definite increases of air volume, was then determined. A
-
50
10
3.5
Figure 1
60 13.5
4.5
0,002
0.04
0.002
0.04
0.006
0.10
