INSTRUMENTATION
Lee Cahn, Inventor, Extended Uses of the Electrobalance Principle by Ralph H. Müller 1
WEEKS ago we started a colA FEW umn on a subject which has engaged our interest and attention for almost 30 years—automatic weighing. As usual one could not get very far in covering recent advances without mentioning the latest ideas and innovations of Lee Cahn and his Electrobalances. Our account was completed when we were stunned to learn, belatedly, t h a t M r . Cahn died on July 3, 1967. We did not know Lee Cahn intimately; our association was technical but we met frequently in far corners of the civilized world, and each occasion was an opportunity for exciting shop talk. We like to think t h a t his career provided a unique example to all who are interested in instrumental advances. I t m a y be instructive to mention some of his attitudes and approaches. Most chemists, and especially analysts, pride themselves on their knowledge of balances, their characteristics, use, calibration, and the skill with which they can attain maximum performance and reliability. Lee Cahn was not a chemist. He was an engineer—about as good as they come! E a r l y British rises of D'Arsonval meter-movements as torque-measuring devices led him to extend the principle to precise weighing, and progressive improvements over the years have resulted in a great array of direct reading, self-balancing, and recording balances currently available from the Calm Instrument Company. They have attained international acceptance, and although they were origi1 Present address, College of Chemistry and Physics, Louisiana State University, Baton Rouge, La. 70803.
Circle No. 44 on Readers' Service Card
nally in the microbalance range, progressive improvements in the torque elements have brought them, with little impairment in sensitivity, into the 100gram range. The history of balance design and manufacture is long, impressive, and well documented. Few chemists would attempt to make radical changes in these well established techniques, but circumstances and special requirements have led to distinct innovations. Calm was responsible for many developments leading to increased speed, convenience of operation, freedom from environmental factors such as temperature, vibration, and atmospheric effects. Curious enough was his constant search for new applications of the Electrobalance principle to problems in which weighing is only an indirect means of measuring some other property. He was constantly aware of the original principle involved—i.e., torque measurement. This led him at once to a most useful and precise method for automatic and continuous surface tension determinations, a skillful adaptation of the Wilhelmy plate technique. A recent development has been the Cahn Density System in which the weight of a plummet immersed in the sample is continuously recorded. One important advantage of this method is the rapid and unambiguous way of determining when temperature and forceequilibrium have been established and the indicated weight of the plummet is to be taken. T h e system has found immediate application in molecular weight measurements with the analytical ultracentrifuge. In this technique one requires the partial specific volume,
obtained as the difference between the densities of the weight of the solute. Differences as small as 9 /xg/ml can be detected. Also the weight of the solute must be known with precision, and this again can be done with the Electrobalance. I t has been known for many years that dilatometric methods can be used to study reaction \'elocitics in solution. Conversely, the continuous recording of solution density as a function of elapsed time ought to furnish the same information. We would guess t h a t the extensive analytical methods studied by Sidney Siggia and his associates could be automated by the Cahn Density System. As Siggia has shown, many binary systems can be analyzed kinetically and, in many cases, where direct chemical methods are difficult or impossible. Automation in this sense would seem to eliminate much of the tedium in following the kinetics. Our personal indebtedness to Lee Cahn was great, if for nothing more than his constant willingness to listen to wild ideas. One could, in his presence, make outrageous extrapolations from current practices. He was sensible and practical enough to realize that these things do not buy next week's groceries or elicit research grants, but they are amusing and stimulating to think about. We are sure that he subscribed to the inventor's dictum— "Never leave well enough alone." I t must be apparent t h a t Lee Cahn surrounded himself with many competant associates and kindred spirits, and t h a t the many accomplishments which he originated or fostered will be continued. VOL. 3 9 , N O . 12, OCTOBER 1967
·
121
A
