This animated film presents the theories of ionization as they are applied to gas filled or vacuum tubes. It was produced by the Division of Visual Arts of the U.S. Office of Education (:14). It opens with a display of a vacuum diode in which electrons are emitted by a cathode (:43) and attracted by a positively charged anode (:44). The number of electrons depend on the amount emitted from the cathode, the potential difference between the anode and the cathode and the negative space charge from the electrons in the stream (:52). The negative space charge limits the flow of the electrons from the cathode to the anode (1:22). In order to reduce it’s limiting effect, a small amount of mercury vapor or an inert gas is added (1:33) making it a gas filled tube (1:41). When the cathode is heated electrons are emitted (1:48). Electrons will return the to cathode if there is no positive potential in the anode (1:56). When a small amount of positive potential is included, electrons will move to the anode (2:08). Electrons move from cathode to anode and atoms of gas move around randomly (2:28). The tube will begin to glow and the current will increase if the anode potential is increased enough (2:40). The tube is said to fire or discharge (2:50). Within the tube, electrons collide with the atoms of gas causing a temporary disturbance of the orbit of the gas atoms (3:05) though they return to normal orbit quickly. Light is released during these collisions (3:35). Some of the electrons moving quickly enough, may force electrons from the atoms (3:51) and these become positive ions (3:59). The electrons removed may then collide with other atoms producing more ions (4:21) in a process is known as ionization (4:29). Ions are attracted to the cathode (4:37), gain electrons and become neutral atoms again (4:42). The negative space charge of the electrons near the cathode limits flow (4:53). A positive presence has the opposite effect (5:01). Each ion counter acts the space charge of the electrons over time, thus neutralizing the negative space charge (5:31). Due to this, two areas develop: the cathode sheath (6:08) and the plasma region (6:15). Simultaneously within the cathode sheath, electrons omitted by the cathode are accelerated towards the anode (6:27) and the electron field accelerates the positive ions towards the cathode (6:38). Neutral atoms within the plasma move slowly (7:07) and electrons exit the plasma through the anode (7:16). The voltage drop across the circuit is small (7:27). If the anode voltage is increased enough to begin ionization of the gas, neutralization of the negative space charge causes an increase (8:06). Due to this, a current limiting device is added (8:23). Adjustment of the resistance changes the current (8:36). To halt the current flow, the anode potential either has to be reduced almost to zero or be made negative (9:06). When an alternating voltage is applied, the anode will be alternately positive and negative (9:25). During the positive half cycle the tube conducts (9:41) and during the negative half cycle there is no current flow (9:45). The addition of a grid to this tube changes it to a gas filled triode also known as a thyratron (10:11). When the grid is provided with a high negative charge the tube cannot conduct when the anode potential is applied (10:21). A negative space charge prevents the flow of electrons across the tube (10:31) and if it is made less negative, electrons will again flow. With further reductions, the electron flow increases (10:51) until the tube fires, gas is ionized, light is emitted and the current increases (11:03). This is accompanied by the formation of the cathode sheath and the plasma region (11:24). A negative grid attracts positive ions (11:34). The sheath of positive ions neutralizes the negative space of the grid (11:41). When the tube fires, the grid can no longer adjust or stop the current (12:05). Before the grid can act as a control again, the anode current has to be stopped (12:30). The thyratron can be used as a rectifier for the addition of an alternating voltage (12:43) though the tube will only be conducting for part of the time. When the grid is made more negative, the tube conducts for smaller portions of time (13:17). The thyratron provides a way to control the current in the anode (13:28). The grid in the gas filled tube is only able to start the current (13:36). A brief summary (13:56) and the end (14:30).

This film is part of the Periscope Film LLC archive, one of the largest historic military, transportation, and aviation stock footage collections in the USA. Entirely film backed, this material is available for licensing in 24p HD and 2k/4k. For more information visit http://www.PeriscopeFilm.com