. Radiography and radio-therapeutics . the vacuum mayreveal itself by a variation in thesounds produced when in action. The best method of detecting re-verse current is by the use of anoscilloscope tube. The construction ofsuch a tube is worthy of aluminium wires, separated by asmall gap, are enclosed in an oblongglass tube of low vacuum, and thewire connected with the negative polebecomes, when the current passes,surrounded by a violet fluorescence. Ifthe current discharges in one direction,only one of these wires shows the violetlight, but if each wire is alternatelynegative
. Radiography and radio-therapeutics . the vacuum mayreveal itself by a variation in thesounds produced when in action. The best method of detecting re-verse current is by the use of anoscilloscope tube. The construction ofsuch a tube is worthy of aluminium wires, separated by asmall gap, are enclosed in an oblongglass tube of low vacuum, and thewire connected with the negative polebecomes, when the current passes,surrounded by a violet fluorescence. Ifthe current discharges in one direction,only one of these wires shows the violetlight, but if each wire is alternatelynegative and positive both wires becomefluorescent and the length of thefluorescent band indicates the intensityof the current, so that we can compare the relative strength of the closingand breaking currents. There are several varieties of tube but the diagram illustrates thegeneral type in use. The instrument is useful. It records the current passing in one direc-tion through the tube. If reverse current is present it represents the differ-. Fio. 75.—Oscilloscope tube in action.(Schall.) (t, The appearance of a tube with thecurrent passing in the right directionwitli a trace of reverse current. /(, Tlie api>earauce with the current passingin botli ilirections in almost equal jiro-portions. SUPPRESSION OF SECONDARY RADIATIONS 87 ence between the two currents. WTien both are equal then no reading isrecorded. If the reverse is greater than the current in the right direction,then it records on the wrong side of the zero mark. When the oscilloscopetube shows that reverse current is present, then valve tubes must be usedto check the reverse. The combination of milliamperemeter, valve, andoscilloscope tubes is a most useful one, helping greatly to regulate theexposures. The X-ray tube affords an excellent indication of the presence of reversecurrent. The change in the appearance of the tube which has reversecurrent passing through it is illustrated in the coloured plate. Secondary Radia
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