AUGUST 15, 1940
ANALYTICAL EDITION
fork being enclosed in a partial vacuum. These units have an accuracy of better than 10 parts per million when they leave the factory. This is very much better than needed and an accuracy of 50 parts per million or 1 art in 20,000 is easily maintained with frequency variations o f *2 cycles and fluctuations of *10 volts in the power supply. This unit was originally developed for manufacturers of timing equipment and uses standard radio tubes which can be obtained a t any radio store. The generator need only be turned on when needed, because factory accuracy is obtained a few seconds after throwing the switch. As can be seen in Figure 2, the generator is a small and compact unit, about 18 inches long, 12 inches deep, and 10 inches high. The Telechron clock across the output of the oscillator is checked against Arlington time signals and a time-rate error of less than 0.3 second per day is obtained. The output of this generator is about 8 watts and, therefore, a special high-torque, low-power motor is required. The motor-driven contactor, operated from the output of the generator, consists of a motor eared to an output speed of 600 r. p. m. In order to obtain goo%,snappy operation on the counters, the breaker points are set so that the circuit is closed for 135' and open for 225" on each revolution. The capacitance of the condenser across the breaker points shown in Figure 1 should be determined experimentally. Various counting devices on the market operate on 110-volt 60-cycle power supply. With alternating current there is, however, a n unfavorable heterodyning effect between the constant-frequency source which drives the motor-driven contactor and the power to the counters. This has the effect
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of supplying varying amounts of power to the counters and, in order to eliminate any possible skipping, the voltage on the counters is controlled to 125 * 2 volts. This is easily done by means of a variable transformer, 3 (Figure 2). The voltmeter, 7 , is connected across the output of the variable transformer and is mounted conveniently near the counters, 4, so that the operator can glance at it from time to time. However, it is felt that direct current-operated counters would eliminate this condition entirely. The use of direct current may introduce a little contact trouble, but with some experimentation the correct condenser across the breaker points will cut this to a minimum. b'ith alternating current, no trouble whatsoever has been experienced with the breaker points and occasionally as many as twenty counters have been operated a t one time. I n order to check the counters against each other, they are started and stopped in pairs. To check against the Telechron clock, the counters are started every minute until six or eight are running simultaneously. Then, after. approximately 10 minutes, the counters are turned off in pairs. This procedure will compensate any error due to the operator. The switches, 5, are well constructed toggle switches, such as are used in regular electrical installations. The ultimate accuracy in any short-period determination on this equipment is *O.l second and in order to bring the timing accuracy to less than *O.l per cent, total efflux times of about 100 seconds or over should be used.
A New Design of Combustion Boat Tongs H. F. PRIEST Williams College, Williamstown, Mass.
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In use, d is turned by means of the knurled knob and as it advances, owing to the threaded portion in e, the bent ends of bb are spread, causing the jaws to grasp the boat firmly. Before insertion in the furnace, the boat is placed on a flat surface, and the tongs are set on it and tightened. It is then slid into the furnace to some predetermined mark on the rod, the jaws are released, and the tongs are withdrawn. The disk, a, guides the tongs. Since the boat is placed in the tongs on a flat surface, it does not drop but is merely released. To remove the boat from the tube the tongs are slid into the tube to the mark and d is turned until the tension indicates that the boat is firmly grasped, when the boat may be easily and safely withdrawn.
a is a brass disk of such diameter as to fit smoothly into the combustion tube. bb are small jaws pivoted on disk a in such a manner that the whole assembly slides smoothly in the tube. When parallel the jaws are spaced a t the width of the combustion boat being used. c is a bar fastened to the bottom of a, and is about twice tis long as the distance to be reached in the tube. The end is turned up as a guide for d. d is a rod fitted with a knob for turning and with a threaded portion passing through a block, e, which is fastened to c. f is a cone on the end of d, against which the short ends of the jaws, bb, fit. g is a coil spring which keeps the short ends of the jams tight against the cone.
The advantages of these tongs over the usual hooked wire are: (1) The boat is held firmly and rigidly while being moved in the combustion tube. (2) The position of the boat in the furnace is easily determined and may be reproduced exactly. (3) Boats are placed in opaque tubes with as great facility as in transparent tubes. (4) The tongs expedite the work in combustion analysis, provided t h a t they are not used in furnaces whose temperature is above 500' C., since at higher temperatures the spring and the brass are affected.
KILLED workers are able to insert a combustion boat into a furnace with great agility, using a hooked wire.
As a n aid to the less skilled worker or in case the combustion boat has to be removed and replaced in the furnace at an exact position during a run, a very simple and useful type of combustion tongs has been designed in the author's laboratory. These tongs have been in use for over a year without upsetting a single combustion boat. The tongs are illustrated in the drawing. The boat is firmly clamped during the time i t is being placed in the combustion tube and being removed from it.
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