Military roentgenology / War Department.

Date:
[1944]
Catalogue details

Licence: Public Domain Mark

Credit: Military roentgenology / War Department. Source: Wellcome Collection.

    45/470 page 35
    42. RECTIFICATION. By rectification an alternat- ing-current and alternating voltage are changed to either a pulsating or direct-current and voltage. Instruments which produce rectification are called rectifiers. The X-ray tube itself may act as a rectifier; otherwise, two principal types of rectifiers are used in roentgen-ray machines. They are valve tubes and mechanical rectifiers. Some valve-tubes are called kenotrons, Figure 28. Wave form, self-rectified circuit. (A) Useful voltage, (B) inverse voltage. MAGNITUDE OF VOLTAGE 43. SELF-RECTIFICATION (fig. 26). A roentgen tube permits electric currents to flow through it, and thus emit roentgen rays, but only if the voltage is applied to the tube in such manner that the anode is charged positively with respect to the cathode; that is, the anode must be positive and the cathode must be negative. Opposite direction of voltage prevents the flow of current (electrons) as long as the heat of the target is below that required for electron emission. (See par. 35c.) The voltage of the secondary coil of the high tension transformer is alternating. Normally, when this a-c voltage is connected directly to a hot cathode tube the current will flow only one-half of the time and, at the same time, one-half cycle of the a-c voltage produces roentgen rays; the other half subjects the tube to high inverse voltage. (See fig. 28.) Current flows through the X-ray tube in pulsations, 30 per second, if the power supply frequency is 60 cycles per second. This is called “half-wave rectification.” All Army Field X-ray Units are self-rectified. 44. VALVE-TUBE RECTIFICATION. Valve tubes (thermionic rectifiers) are in some respects similar to roentgen ray tubes. (See fig. 29.) They consist of a highly evacuated glass tube with a tungsten filament mounted in one end and a molybdenum plate mounted in the other. The filament 1s heated by a-c current to approximately 2,000° C., from the secondary of a filament transformer. As high tension voltage is applied to the filament and the plate (the filament being negative with respect to the plate) the current flows through the valve. No current flows if the voltage is applied in the opposite direc- tion. Thus, alternating-currents are rectified by valve tubes. A valve tube, like a roentgen-ray tube, has a saturation current—but valve tubes are normally operated under these saturation currents. Thus, increases of applied voltages result in increase of valve tube current. The voltage drop across a valve tube should not exceed a few hundred volts if filament temperatures are maintained at high values. Manufacturers specify proper heating cur- rents for the valve tube filaments. If these currents are permitted to fall below the specified amperage, potential difference across the tube may exceed saturation values, and thus, result in the emission of roentgen rays. Such emission is undesirable and dangerous. No means of rapid cooling of the valve tube plate is provided; melting temperatures may easily be attained. Since roentgen ray emission Is always possible, valve tubes should be located where the patients and technicians are not exposed to them. 45. INVERSE SUPPRESSOR. In the case of certain relatively low milliamperage capacity units (10 to 30 milliamperes), instead of incorporating valve tubes in the high tension circuit for the purpose of rectification, an Inverse suppressor may be con- nected into the primary circuit of the high tension transformer. The purpose of inverse suppressors Is to limit partially the amplitude of voltage effective upon the target of the X-ray tube. Inverse sup- pressors may be constructed on the principle of a
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