Daniel 1. Harris
The University of Chicago Chicago, Illinois
A Rapid, Direct-Reading Micro-Balance
The balance described here has proved useful for rapid, routine weighings in the range 0-130 mg where an accuracy of not more than k0.25 mg is sufficient. It is easily and cheaply constructed, is direct-reading, and damps in about five seconds. The photograph shows the essential parts of the balance. It consists of a "light" spring from a Jolly specific-gravity balance, a stiff suspension wire on which a cross-hair is mounted, pan, and oil damper. A sharply convergent beam of light from a microscope lamp is focused on the cross-hair, and the shadow is projected t,hrough a simple lens system onto a calibrated
screen. A glass tube, 35 X 550 mm, protects the spring from air currents. The spring is hung from a cross bar sealed t o the top of the glass tube, and a beaker inverted over the glass tube to exclude dirt. It is obvious that glass tube, damping pot, etc., must he rigidly fixed. Forceps are used to clamp the suspension wire when adding or removing a sample. The damper consists of an aluminum cylinder, open a t both ends, made from a 35-mm Ansco color-film can; it is suspended in a light mineral oil. This arrangement gives maximum horizontal damping and minimum vertical damping. The lens was constructed from three simple hands lenses and gives a magnification a t one meter of about 40 times; a condenser from a compound microscope could equally well be used. The response of the balance is linear. Neither variation in room temperature nor heat given off by the microscope lamp significant~ly affect t.he pcrformance. I n several years use, the calibration has not changed. It is advisable to add and remove a 100 mg tare weight to the balance pan hefore starting a series of weighings. This presumably wets the damper suspension wire uniformly. The zero point can then be set by adjusting the scale vertically. This application of ancient principles has proved useful to us; it may also to others. A similar balance with onefifth the sensitivit,~and five times the range can be constructed from the "heavy" spring of the Jolly balance.
Volume 38, Number 9, September 1961
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469
