Abstract
The determination of the threshold of vibratory acuity is a valuable part of the neurological examination, particularly in the diagnosis of lesions affecting the posterior columns of the spinal cord. There have been numerous studies in which an attempt has been made to obtain a quantitative measure of acuity. The tests have all been made with a tuning fork of some description, usually with some device designed to measure the intensity of the vibration, an end which, in the opinion of the workers themselves, was not satisfactorily attained.
We have, therefore, constructed an instrument which produces a vibration of constant frequency, the intensity of which can be varied over a considerable range, and measured accurately. It consists of an iron pole-piece which constitutes the vibrating member, and to which is attached a round, metal button 12.5 mm. in diameter which is placed over the bony prominence to be tested. This pole-piece is actuated by an electromagnet energized by an alternating current of 60 cycles per second. By means of a potentiometer this current may be varied, and its strength measured by means of an alternating current milliameter in series with the magnet winding. Thus the amplitude of vibration, roughly proportional to the current flowing through the electromagnet, is subject to control and measurement. Since the intensity of vibration bears no absolute relation to the amperage, depending on the design of the coil in the particular electromagnet, this intensity is expressed in arbitrary units from 0 to 10. Each individual instrument should, therefore, be calibrated to correspond to the original one used as a standard. An even pressure is maintained by allowing the instrument to rest by its own weight over the bony prominence, the operator merely supporting the end of the handle.
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