The Faraday Magnetic Susceptibility System: Two Basic Improvements The Faraday method of obtaining magnetic susceptibilities is a rather general technique used by many inorganic chemistry laboratories. A good resolution system which will yield three significant figures is composed of a current regulated power supply, a water cooled 4-in. electromacnet with H capabilities of about 7000 G, tapered pole pieces to eive a constent d ~ / d ; - o n a perpendicular plane, a microbalance sensiti;e to changes in weight of about 1 x lo-: g, and several other less imoartant. hut still essential oieces. sueh as a stand for the balance.. bell iar and evacuatine .. svstem. a hane d0u.n film fL>kthe h&ket, standard weiiht5, good qualit). quartz buckets, rolling baw for the magnet, ctr. 1.a;t. hut di;; very important, static charge b~tweenthe bucket and fihpr and the side* of the wid finger ran, notwithstanding all the good equipment mentioned above, play havoc with the experiments. In this note, we present some basic improvements on the hang down fiber and a simple method to solve the static electricity problem. These improvements have allowed us t o carry out susceptibility measurements more accurately, in a more effortless and continuous manner. H a n g Down Fiber. The usual hang down fibers are about 90 cm in length, weigh about 100 mg, and are made out of good quality quartz. This type of fiher has some disadvantages: 1) quartz is rather fragile and it is somewhat tedious to prepare the right length and weight required; 2) quartz has in certain cases paramagnetic impurities, g values of about 2 in the epr spectrum, which introduce small errors a t low temperature, and 3) the density of quartz is about 2.2 glcm." We have recently started using some manofilament Nylon line, 1lh test, which obviates all the previous difficulties. This Nylon fiber is shatter-proof and easily cut and tied to the desired length. Nylon is completely diamagnetic and has a smaller density than quartz, about 1.5 g/cm3, with a total weight of the fiher of about 15 mg. Since the accuracy of the microhalances are inversely proportional t o the total pan weight, the use of this Nylon fiher also reduces the total systematic error of the experiment. Finally, the Nylon fiher did not stretch on loads up t o 500 mg, well within the usual operating range for this experiment. and did not chanee in lenpth - hv.immersine in liouid nitroeen. Our limit of detection of chanees in leneth is *0.05 mm. For experiments a t temperatures above roo; temperature,< small section of quartz fiher &be used between the hueket and the Nylon fiher because the latter will shrink considerably and nonelastically a t elevated temperatures. However, a t all other temperatures, the Nylon fiber was perfectly stable. Static Electricity. Even in temperature controlled rooms, the relative humidity tends t o go well below 50% on dry days. This, coupled with normal handling of the equipment, introduces a noticeable amount of static charge in the hueket and fiber. This potential difference between the cold finger in the outside and the hang down fiher and hueket imthe inside, force the hang downs to move close to the eold finger and interrupt the weighings. In more extreme eases, the whole experiment has t o be stopped. In order t o avoid these difficulties, we made up two circular 8 sources.The approximate diameters of these radioaetive sources were sueh that one could be introduced to the bottom of the eold finger (below the bucket) and the other could he permanently attached to the outside of the cold finger, about 1em higher than the bucket. The sources eonsisted of 13'Cs, which gives reasonably dense ionization of the air around it without needing the delicate thin windows required by an a emitter. Each source had a strength of about 100 pCi. They were made by evaporating to dryness '"CsCI (0.5 N in HCI) on the adhesive side of a piece of plastic adhesive tape and sealing the material in place with a second layer of tape. These samples withstand normal handling with relatively low probability of leakage. Both of these sources were mounted on the cold finger and left there since their preparation. We have obtained room temperature, 195*K, and 77°K data on about 20 compounds with no signs of static electricity problems. Acknowledgment. We are endehted t o Drs. Jack D. Ullman and Charles E. Engelke for discussions and the preparation of the sources, also, to the Research Foundation of the City University of New York for financial support. H. A. Lehman College of CUNY Juao F. Villa Harold C. Nelson Bronx, New York 10468
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28 1 Journal of Chemical Education
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