John T. Stock ond M. A. Fill
University of Connecticut Storrs
An Aid to Weighing by Swings
T o get maximum balance performance, weighiug must be done by the method of swings. To obtain swings of optimum magnitude by mere release of the arrestment requires some skill or several attempts. Fanning with the hand is simple, but a beginner can cause serious damage if he strikes the moving parts. A useful alternative is to make a single careful release and to initiate or control the swings with the balance case closed.',2 This may be done quite simply by the electromagnetic device shown in Figure 1. Construction of such a device affords a simple student project.
Figure 1.
Solenoid and control drcuit.
Armature A is made by plaiting together six 20-em lengths of 36-gauge soft iron wire. A 5-turn coil about 5 mm in diameter and 10 mm long is then formed on one end, while the other is twisted around the right hand outer pan support rod as shown. Acting as a compensator, an ideutical armature is similarly attached to the left hand pan; the zero of the balance is then adjnsted. Solenoid B has about 500 turns of 28-gauge insulated copper wire, the ends of which are soldered to subminiature flexible connecting leads. The solenoid may be wound on a short length of corkborer or other thin brass tubing about 15 mm in diameter and enclosed within a pair of plastic bottle-caps placed mouth-tomouth. With some balances, the flexible leads may be passed out of the case through the hole in the baseplate provided to operate the pan-arresting levers. The leads are connected to a single penlight cell through a 'STOCK,J. T.,A N D FILL,M . A,, Melallurv'a, 40, 232 (1949).
' VOLLRATH,R. E., Ind. Eng. Chern., Anal. Ed., 13,564 (1941).
366
/
pushhutton C and a preset resistor D of maximum resistance about 25 ohms. These parts are mounted within a small flat box so that the pushbutton may be easily operated. The solenoid housing rests upon, or is lightly cemented to, the balance baseplate so that the solenoid is coaxial with the right-hand armature. With the arrestment released and the pointer a t mid-scale, about one turn of the armature should he outside the solenoid, as shown. Continuous depression of the pushbutton then causes the pointer to travel smoothly across the scale to a quite reproducible limit determined by the resistor setting, as indicated by curve I, Figure 2. This limit is but slightly affected by loading the balance; a comparison of curves I1 and 111 appears to indicate that the change is mainly due to the decreased sensitivity of the loaded balance.
Jeurnol of Chemical Education
Figure 2. Relotionship between resistor selling or load and steady deRection. Curve I (upper scale), unloaded balance; ll (middle scale), resistor set a t zero; Ill Ilower scole), sensitivity of balance, diviiiom pee mg.
In practice, the pushbutton isusually briefly depressed to augment or diminish the swing resulting from normal arrestment release. I t is obvious that completion of the circuit when the armature is receding from the solenoid will reduce unduly large swings. The maximum current is less than 0.1 amp and is drawn for a few seconds at a time. Replacement of the cell thus depends mainly on shelf life.
