August IS, 1941
565
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 an 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 an 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 deflectsit.
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Base
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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-
