Sphere Gap

Abstract

HomeRadiologyVol. 5, No. 4 PreviousNext EditorialSphere GapPublished Online:Oct 1 1925https://doi.org/10.1148/5.4.349MoreSectionsPDF ToolsImage ViewerAdd to favoritesCiteTrack CitationsPermissionsReprints ShareShare onFacebookTwitterLinked In AbstractWhy is the sphere gap the standard for measurement of voltage in medical practice?What are the reasons against the use of point gap for measurement of voltage?What are the general specifications under which the sphere gap should be built?These three questions come up frequently whenever roentgenologists are gathered known together, and although they have been discussed from time to time in technical literature, perhaps it would be interesting and timely to give such direct answers to the questions as are set forth by our friends, the physicists.The sphere gap has been standardized for measurement of voltage in medical practice because it is necessary to record, collate and transmit, with the maximum accuracy, information which has to do with the practice of medicine and the welfare of the human body. In the use of X-rays, either for therapy or radiography, the factor of voltage has a direct and vital influence on both the quantity and quality of radiation produced by the tube. It is, therefore, doubly important that the record and transmission of correct voltage should be done with all possible care and accuracy.In adopting the sphere gap for measurement of voltage, we are following a rule established by the Standards Committee of the American Institute of Electrical Engineers. That rule authorizes the use of a point spark gap for measurements of voltages up to 70 kilovolts, maximum, provided the point spark gap is built and used in accordance with certain standard specifications.For measurements of voltages above 70 kilovolts, maximum, they prescribe the use of the sphere gap, with the statement: “When used as specified, the accuracy obtainable should be approximately 2 per cent.” It might be noted here that tests made recently at Harvard University show an error of even less than 2 per cent in measurements made by means of a standard commercial sphere gap.The answer to the first question is, then, that the sphere, gap is the most accurate known method of measurement of an important factor in medical work and it is, therefore, perfectly logical that it should have been adopted as the standard in medical practice.As a reason against the use of the point gap, it is perhaps not sufficient to cite the rule of the A.I.E.E., because at once the question would be asked: Why have they made such a rule? There are several reasons against the use of the point gap, not the least of which is the difficulty of constructing and maintaining such a device in accordance with the specifications necessary in order to obtain reasonable accuracy, even for voltages so high as 70 K.V., maximum. For instance, “The needle spark gap should be between new sewing needles, supported axially at the ends of linear conductors, which are at least twice the length of the gap. There must be a clear space around the gap for a radius at least twice the gap length.” It is quite evident that that specification, means plenty of room and new sharp needles for each measurement.Article HistoryPublished in print: Oct 1925 FiguresReferencesRelatedDetailsRecommended Articles RSNA Education Exhibits RSNA Case Collection Vol. 5, No. 4 Metrics Altmetric Score PDF download