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INDUSTRIAL AND ENGINEERING CHEMISTRY
Figure 1shows the details of construction of the tangentimeter. A , A are two pieces of transparent Lucite, 2 inches (5.0 em.) wide and 0.375 inch (0.95 cm.) thick, and of any desired length. The ends of the plates should be highly polished to make them completely transparent. B is a metal mirror 2 inches (5.0 cm.) wide and 1inch (2.54 cm.) high, which is set upright between the ends of the two Lucite plates and is soldered into shallow slots cut in the brass binding strips, C , C. These strips are 0.375 inch (0.95 cm.) wide and 0.06 inch (0.16 cm.) thick. The binding strips carrying the metal mirror are fastened to the plates by means of small machine screws, D, which fit into holes drilled and ta ped in the Lucite plates. E, E is an index line scribed on the rower side of the lates, equidistant from the edges and exactly perpendicular to t i e metal mirror. The line may be easily scribed on the soft surface of the plastic by means of a machinist’s scriber, a narrow engraving tool, or a sharp needle. Figure 2 shows a photograph of the instrument in use. The tangentimeter is placed on the curve in such a position that the
Vol. 13, No. 8
intersection of the index line and the mirror coincides with the point on the curve at which the tangent is to be located. The instrument is rotated about this point until the portion of the curve in front of the mirror and its reflection in the mirror appear as a continuous, smooth, unbroken line. When this position is obtained, it will be seen that the index line on the instrument is exactly tangent to the curve. The intercepts of the tan ent line will be easily visible, and the slope may be very simply cafculated. This instrument has been used in this institution for the past year, both for research work and for student computations, with pronounced success.
Literature Cited (1) Latishaw, J . Am. Chem. Soc., 47, 793 (1925). (2) Richards and Roope, Science, 71, 290 (1930).
A Device for Starting the Swinging of a Balance RICHARD E. VOLLRATH, Allan Hancock Foundation, University of Southern California, Los Angeles, Calif.
S
EVERAL methods are in use for starting the swinging of a balance in precise weighing. A current of air may be produced near one of the pans by a fanning motion of the hand, one of the pans may be pushed down by means of a fiber such as horsehair attached to a matchstick, or the beam may be given a push by means of a fiber attached to the rod carrying the rider. Guthrie (3) and Arzberger (1) have suggested that a puff of air be directed against the underside of one of the pans by squeezing a rubber bulb connected to a tube projecting through the base underneath the pan. None of these methods is altogether satisfactory, because the amplitude of the swing cannot be conveniently controlled. Furthermore, in the first two methods the balance case must be opened, and this is time-consuming and obviously unsatisfactory when hygroscopic objects are weighed. The writer has used for some time a simple and convenient device which makes i t possible t o start the beam swinging without opening the case and to control the amplitude evzn while the beam is swinging. There are no references to previous use of the device except a rather obscure statement (1, 2) in a work on balances. The present device makes use of the pull of a small electromagnet on a piece of soft iron or steel mounted on the beam or the pans of the balance. When
rminals
F I ~ U R1.E DETAILS OF ELECTROMAGKET
the electromagnet is excited by closing a switch for a few seconds the pull exerted deflects the beam and starts it swinging. The balance used by the writer is a type T analytical balance made by Wm. Ainsworth & Sons and has a sensitivity of 0.05 mg. at a load of 200 grams. It was furnished by the maker with certain minor changes to permit the mounting of the device described below: a 0.79-cm. (6/la-inch) hole drilled in the black glass base just below the center of the left-hand pan for mounting the electromagnet, a 0.62-cm. (0.25-inch) hole in the front of the case for mounting a small push-button switch, and steel buttons on the underside of the pans in place of the aluminum buttons against which the pan arrests press in this make of balance. The steel buttons were plated to avoid possible corrosion. I n Figure 1 is shown the electromagnet mounted under the base of the balance and concentric with the center of the left pan, the core projecting through the hole in the base. A brass nut screwed on the threaded upper end of the magnet serves to hold it in place. The core of the electromagnet and the spool on which the exciting winding is wound are all in one piece turned out of cold-rolled steel bar. The spool is wound with about 350 turns of number 25 double cotton-covered magnet wire, impregnated with shellac and baked dry. The ends of the winding are soldered to lugs on a strip of Bakelite to facilitate connecting the coil into the exciting circuit. The presence of the stiffening rib indicated in Figure 1 necessitated making the magnet core longer than would otherwise have been necessary. The distance between the upper end of the core and the pan arrest was about 0.62 cm. (0.25-inch)-large enough to allow the pan arrest to be locked in the down position as indicated by the dotted lines of Figure 1. The drawer in the base had a notch cut out of the back end, to allow it to pass the coil which projects into the drawer compartment. The push-button switch was made to operate with slight pressure and is shown in Figure 2. It consists of two strips, A and B , of 28-gage spring phosphor bronze 0.62-em. (0.25-inch) wide with a contact point of copper on one of them. These sprin s are mounted on a strip of Bakelite which is clamped to the insife of the balance case by means of a nut that screws on the bushing in which the brass rod of the push button slides. A slight pressure on the push button pushes the two springs into electrical contact, thereby allowing the energizin current to flow through the electromagnet, as may be seen &om the wiring diagram (Figure 3). The small Bakelite knob on the end of the push rod near the spring prevents the metal parts of the balance from coming into electrical contact with the circuit when the push button is operated. The electrical wiring is concealed in the space below the base and to the left of the drawer. A lamp cord passes through a small hole in the back of the balance and is connected to a 50-watt lamp serving as a current-limiting resistor and a plug for connecting to the 110-volt line. The most convenient place for the push button is to the left of the drawer. The weighing is carried out in the usual way until balance is established closely enough to permit final determination of the
August IS, 1941
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ANAL:YTICAL E D I T I O N
50-W Lamp
Careful tests with precision weights were made to determine possibleerrors that could be attributed to residual magnetism in the core of the electromagnet. This I IO-V.(A.C) was done by weighing an object of 4 Switch accurately known weight first without the magnet in place and then with the magnet in place. No errors as great as the sensitivity of the balance (0.05 mg.) could be Magnet Coil detected. However, errors could FIGURE 3. WIRING DIAGRAM have been eliminated by using a solenoid without iron, in which case a somewhat longer coil with more turns of wire would have served a s well. The device may also be applied to other balances. For example, a deflecting force was applied to the steel pointer of a n old balance by means of a small electromagnet mounted on the column of the balance by a clamping arrangement. If the small weight usually fastened to the pointer for adjustInch ing the center of gravity is made of iron, a pull can be exerted on this by a n electromagnet mounted on the column. Another possibility, not tried as yet, is to use a flat solenoid of FIGURIZ 2. DETAILS OF PUSH-BUTTON rather large diameter mounted underneath one of the pans SWITCH and fed with alternating current. This might be expected t o result in a pulI on the pan because of the eddy currents in’ duced in it. point of equilibrium by the method of swings. With the balance closed, the beam is released and then the pans are released. To Literature Cited start the. beam swinging the switch is closed long enough t o give a deflection of 5 or 10 divisions. The switch is then opened and (1) Felgentraeger, ,6FeineWaagen, wLgungen und Gewichte,,,p. 12, the beam allowed t o swing freely. The amplitude of the deBerlin, Julius Springer, 1932. flection may be accurately adjusted by controlling the length (2) Ibid,, p. 172. of time the switch is kept closed. The beam can be brought Guthrie, Nature, 121, 745 (1928). to rest by closing the switch for a short time just as the pointer CONTRIBUTIONs t o Physics, NO.1. returns t o zero from the side toward which the magnet deflects it.
Base
A-l
d
Magnetic Stirrer for an Evacuated Sorption Apparatus SAMUEL A. WOODRUFF AND ALFRED J. STAMM, Forest Products Laboratory, Madison, Wis.
I
N ORDER to ensure that the vapor over a liquid is truly
in equilibrium with the liquid, it is important to break the surface of the liquid continually, even in a n evacuated system. Violent stirring is not desired because any liquid spray may cause a supersaturated condition. The simple device shown in Figure 1 met these requirements admirably. The glass bulb, B, containing the liquid t o be stirred, has a heavy platinum wire support, P,sealed as shown into the wide neck of the bulb. This serves as the support for the stirring addle, 8, which is made from a glass rod flattened at the end. $he upper part of the stirrer consists of a glass tube into which an iron nail, N , is sealed. The upper and lower parts of the stirrer are connected through a heavy platinum wire which is looped around the platinum support, P. The stirrer thus oscillates freely about P as an axis. M is a small electromagnet coil taken from an old electric doorbell. This is connected t o the secondary coil of a doorbell transformer (18 volts). The 110volt primary coil of the transformer is connected in series with a 20- to &watt lamp, in the socket of which is placed a 60-watt 120-volt “flasher button”, a device for thermally making and breaking the circuit once or twice a second. The intermittent current through the primary of the transformer causes a similar intermittent current to flow through the secondary of the transformer and through the electromagnet. N is thus pulled over t o M and released once or twice a second, causing S to oscillate slowlp within the liquid.
FIGURE1.
INTERNAL STIRRER FOR A N TION APPARATUS
EVACUATED SORI-
