A recently developed method of determining the number and size of particles suspended in a conducting solution is to pump the suspension through a small orifice having an immersed electrode on each side to supply electrical current. The current changes due to the passage of particles of resistivity different from that of the solution. Theoretical expressions are developed which relate the current change caused by such particles to their volume and shape. It is found that most biological cells may be treated as dielectric particles whose capacitive effects are negligible. Electrolytic tank measurements on models confirm the theoretical development, and electric field plots of model orifices are used to predict the observed pulse shapes. An equivalent circuit of the orifice-electrode system is analyzed and shows that the current pulse may be made conductivity-independent when observed with a zero input impedance amplifier.
FOR MANY years the appreciation of vibration has been used in the study and diagnosis of various neurologic disorders.1 Up to the present time both the tuning fork and electrical vibrators 2 have been used for such determinations. However, while the latter represent a vast improvement over the former, the attendant errors in both have restricted their use to disorders that produce only large changes in the vibratory threshold. Furthermore, such instruments have been incapable of absolute calibration and as a result have been limited to relative studies. Recent investigations on certain neurologic disorders 3 and the effect of analgesics and other drugs have again served to emphasize the importance of the vibratory threshold, and it is the purpose of this paper to describe an instrument and a technique that allow the precise measurement of this threshold on an absolute scale.
PHYSICAL DESCRIPTIONWhile it is not the purpose of this paper to discuss either the modality or the particular mechanism involved in the perception of vibration, it is obvious that any technique of measurement must involve some sort of a vibrating body held against the subject. In the case of the tuning fork, it has been standard practice to start the fork at a given amplitude of vibration and to measure the time for the vibratory sensation to disappear. Since such a time decay depends on both the initial amplitude and the logarithmic decrement of the fork, it is obvious that the errors involved are enormous, even for an experienced operator using the same fork on the same test spot. Under these circumstances, errors in measure¬ ment may be on the order of 50 to 100%. and under some circumstances of com¬ parative measurements errors on the order of 300% are not unusual.Another complaint against the tuning fork is that the time of decay to a threshold bears no linear relation to the amplitude, so that comparative measurements are From the Case Institute of Technology.
A jet 'directer' is described which permits particles to be directed along selected trajectories through a Coulter orifice. The resulting current waveshapes are seen to be characteristic of the paths taken by the particles. No evidence of the effect of the motion of charged particles on the waveshapes was detected which supports the theoretical prediction of the small magnitude of this effect. The waveshapes from known paths, when examined in the light of measured or calculated electric fields in these orifices, support a direct relationship between the resistive component of the instantaneous current amplitude and the magnitude of the local electric field at the position of the particle. I n short Coulter orifices a region of uniform axial electric field is found midway between the orifice axis and the edge. Neither the axis nor the edge has regions of axial uniformity. Only spheres sized while traversing the region of uniform field will produce a true size distribution in these short orifices.
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