Text: THE ELECTRIC SPARK. When a small potential difference is impressed on two electrodes in air a very small current will flow which will be proportional to the voltage. As the voltage is increased the current will increase in proportion for a short while only, and then will assume nearly a steady value over a wide range of voltage changes. This steady value, A in Fig. 1, is the saturation current. As the voltage is still further increased, a point is reached at which the current again begins to rise, at first slowly, then very rapidly as the sparking voltage is reached. The saturation current is the value.of current when all of the ions in the field between the two electrodes are acting as carriers Then in order that the current may increase past point A, Fig. I, additional ions must be created is some way. This is accounted for as follows. As the voltage between the two electrodes increases the force acting to attract the ions in the field increases correspondingly, and the ions travel with greater speed toward the electrodes. When they collide with other ions and molecules while on their way to the electrodes, greater forces take place in the collisions and new ions are formed the number of new ions increasing with the voltage. Finally with large voltage their speed becomes so great as to result in almost a complete breakdown of all the molecules in the field. At this stage the spark passes. The sparking voltage, which is defined as the lowest voltage that will cause a spark to pass between two given electrodes, depends upon many factors, among them being the shape of the electrodes, the distance between the electrodes, the atmospheric pressure, the humidity or amount of moisture in the air, and the amount of light that is admitted to the sparking space. The sharper the electrodes the lower the sparking voltage. The sparking voltage varies almost directly as the pressure, and the sparking voltage increases with the distance between the electrodes, although the relation is not a linear one Sparks will pass at a lower voltage in daylight than in the dark. This is because the light helps some in creating ions. Whenever a spark passes between two electrodes it is accompanied by a loud crackling sound. This sound is due to the pressure of the spark. The ions in the field of the electrodes acquire considerable kinetic energy. Pressure is proportional to the kinetic energy per unit volume. The actual volume of the spark is very small so that the ratio of kinetic energy to volume will be high, and therefore the pressure. Pressures of over 100 atmospheres have been measured in some sparks in air. THE ELECTRIC ARC. With the spark the voltage is usually several hundred volts and the current is small. With the arc the voltage is small and the current may be several amperes. In the carbon arc the temperature of the positive terminal is much higher than that of the negative, the positive being about 3,500° C. and the negative 2,700° C. The temperature of the arc itself is higher than that of either electrode. After an arc has been in operation for some time the positive electrode becomes hollowed out on the end, a crater being formed, while the negative becomes pointed. Both electrodes lose material, the positive wearing out faster. With some electrodes the arc will be intermittent, and if the terminals are of different material the voltage may depend on the direction of the current, especially with electrodes of carbon and a metal; The arc passes much more easily with the carbon negative and the metal positive. Some metals are termed non-arcing, meaning that they are not heat maintaining'and tend to make the arc go out. It is hard to get a good arc in hydrogen gas with any metal, and no arc can be maintained in nitrogen between silver electrodes. There are some interesting applications of the facts outlined above in connection with the design of the radio arc transmitter. When the arc operates at radio frequencies it is necessary to have it make and break several hundred thousand times per second, and consequently substances that maintain an arc easily cannot be used. The Federal Telegraph Co. uses hydrogen gas in some of their arc chambers, and uses large electromagnets in order to make the arc break quickly. Up to date arcs have not been made to work satisfactorily much below 3,000 meters, the frequency at this wave-length being 100,000 cycles per second. The Federal Co. uses 3,100 meters as the commercial.wave-length for their ship stations while the Navy uses 3,000 meters as the lowest. THE COMPRESSED AIR CONDENSER. This condenser consists of a number of plates arranged as in the common air condenser, the whole being put in a container that will stand very high pressure. The condenser is then subjected to high air pressure, the object being to retain the advantages of the air condenser and still be able to'apply high voltage to it. The spark potential between two plates in air varies almost directly as the pressure, so that with the compressed air condenser large voltages may be applied without breakdown of the dielectric. At the same time the leakage between the two plates varies directly as the pressure so that losses increase with the pressure from that source. Ionization in a gas is less at high pressure than at low so that corona from sharp edges, etc. will be greatly reduced. Taken altogether the compressed air condenser is very good electrically, its chief disadvantage being its bulkiness. THE QUENCHED SPARE GAP. The ideal spark gap is one that will be a perfect conductor while the spark is passing and a perfect insulator immediately after the spark breaks. The passage of the spark creates a good many ions in the field between the electrodes and while these remain the gap will be a poor insulator and will break down at potentials much lower than it is supposed to. These ions can be eliminated either by an air blast on a stationary gap, or by circulation of air in a rotary gap. A combined rotary gap and air blast gets the ions out of the way quickly, but the air blast is cumbersome and unsatisfactory. A quenched gap consists of a series of very short gaps, about .01 inch each, in series, the gaps being in air tight chambers and the electrodes being of heavy metal with silver at the sparking surface. The electrodes must be kept cool. When the spark passes ions are produced. The ions are attracted to the cold metal surfaces and discharged by them before the next spark takes place. The first few sparks eliminate all the oxygen f rom the chamber by combining it with the silver electrodes to form a thin film of silver oxide over the surface of the electrodes. This leaves only nitrogen in the sparking space, and it is impossible to maintain an arc in nitrogen between silver electrodes so the spark is quickly quenched out. The electrodes are kept cool with a fan. If they are allowed to get hot they may reach the ionizing temperature and then the quenching action of the gap will cease. Temperatures as high as 100° Centigrade are allowable. Copper electrodes can be used instead of silver but danger of arcing is greater. Other metals besides copper and silver all maintain arcs in nitrogen easier than in air, and hence cannot be used. THE VACUUM TUBE. There are many important applications of the principles outlined in this paper in connection with the design of vacuum tubes. A vacuum tube consists essentially of three elements, filament, grid, and plate, arranged in that order and enclosed in an evacuated glass container. The device is too well known to necessitate a physical description. The filament is heated to incandescence by passing an electric current through it. At this temperature the kinetic energy of the negative particles becomes so great that some of them acquire sufficient velocity to break through the surface tension of the metal and are projected out into the space around the wire. If the wire was formerly electrically neutral this loss of negative particles has the effect of producing a positive charge on the filament and an attraction is immediately set up between this positive charge and the negative charges on the particles. This force tends to draw them back into the wire. If now a positive charge is put on the plate the particles which leave the filament arc subjected to two attractions, one back to the filament and the other to the plate. There will obviously be some point between the filament and plate at which these two attractions will be equal in magnitude and opposite in direction. In the ordinary vacuum tube this "point" will be a plane parallel to the plate and situated between the plate and filament. The higher the plate voltage the nearer this plane of equal attraction will be to the filament. Every projected particle from the filament that crosses this plane will continue to the plate and every one that does not cross it will he drawn back to the filament. The nearer the plane is to the filament the more particles will cross it, and the larger the plate current will be. When the position of the plane is very close to the filament a large increase in plate voltage should be required to make any appreciable change in the position of the plane and therefore in plate current, so that the plate current-plate voltage curve should flatten out when the plate voltage gets high. The value of plate current in the flat part of the curve is called the saturation current. The saturation current depends only upon the number of particles shot off from the filament. For a given filament temperature then the value of this saturation current is independent of the spacing between plate and filament, but with large spacing more plate voltage must be applied to reach the saturation point. The grid when charged positively acts in a similar manner to the plate and its action for a given condition may be predicted from the same rules. When a positive charge is put on the grid a small current flows between the grid and filament, but at the same time instead of robbing the plate of current the plate current shows an in-crease. This is hard to understand at first glance but may be explained as follows. A positive charge on the grid has the effect of aiding the field of the plate and of pushing the plane of equal attraction nearer to the filament if it is not already at the Saturation point. This makes an increase in the number of particles that start in the general direction of the plate and grid. The grid consists of a number of spaced wires all having the same electric charge by reason of being connected together. When a negative particle comes near the grid it is attracted by the charges on all the wires, and as it gets very near will be attracted by the two nearest grid wires. If the path of the particle is such that it is directed exactly between two grid wires, the attraction from each will be equal and the two will counterbalance each other. The particle will go on straight through due to the attraction of the plate. If the path of the particle is directed somewhat nearer one wire than the other it will be drawn in the direction of the nearer wire, but even then the two attractions will neutralize partly and the path of the particle will be a curve, towards the nearer wire till it gets between the two and then straight to the plate. (See Fig. 2.) Only in case the particle is directed almost at a grid wire will it be drawn into the grid. The increase in plate current is therefore apparent. When a negative charge is put on the grid its effect is to counteract part of the grid, its effect is to counteract part of the positive charge on the plate, move the plane of equal attraction out from the filament and decrease the plate to filament current. In some types of radio apparatus, particularly in radio telephones, it is desirable to have no leakage from the grid through the tube with a negative charge on the grid and this is one reason for using an oxide filament in these tubes. An incandescent oxide will not discharge a negatively electrified body.

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