International projectionist (Jan-Dec 1945)

flow in any circuit which contains a condenser. As we shall see a little later, alternating current will allow current to flow in a circuit that contains a condenser. A condenser is very useful in radio circuits for separating direct currents from alternating currents. A condenser has a smoothing effect when placed across a direct curient that is varying slightly in amplitude (pulsating direct current) and these condensers are commonly used in the power supply section of radio receivers. In this application, they are known as filter condensers. When an exciter lamp in a motion picture projector is operated from rectified alternating current, a great deal of filtering is necessary in order to remove the 60 cycle hum which would show up in the sound. Large condensers are used for this purpose. Whenever circuits which contain a large amount of inductance are opened, a great deal of sparking results at the switch contacts. This is true of direct current circuits which feed motors, generators, spark coils, relays, and automobile ignition systems. Since inductance tends to keep the current flowing when the switch is opened, a hot spot develops on the switch contacts. This hot spot burns the contacts and carries away from them small particles of copper. This action results in premature wear on the switch and lowers its efficiency as a switch because of pitted contacts. If a condenser is placed across the switch contacts the condition is soon remedied. The reason for this is that the energy set flowing by the inductance in the circuit charges the condenser and eliminates sparking. Storage of Energy in a Condenser The amount of energy that the dielectric field of a condenser can store depends upon the size of the condenser — the larger the condenser, the greater the energy. TABLE OF DIELECTRIC CONSTANTS The following is a table of approximate dielectric constants for some materials that are commonly used as insulators in condensers. Celluloid, photographic film. 6.7 Cellulose nitrate 3.8 Fiber 4.8 Fused quartz 4.0 Glass 6.5 Pyrex 4.9 Hard rubber 3.0 Isolantite 6.1 Mica. India 6.0 to 8.69 Paper 2.6 Polystyrene 2.6 Porcelain 7.0 Wood 2.5 to 6.8 The size of a condenser depends upon four factors: (1) the area of the plates lacing each other; (2) the number of plates that are connected in parallel; (3) the distance between the plates or the thickness of the dielectric, and (4 1 the type of dielectric used. If the quantities in the first and second factors listed are increased, the capacitance of the condenser will increase as will its ability to store energy. If the distance between the plates were increased, the capacitance would decrease because the concentration of the dielectric field would be less dense. The type of dielectric used in a condenser is very important in determining its size. The atomic structure of some dielectrics is such that they aid the dielectric field more than others when an electrical pressure acts upon them. This quality has been designated as the dielectric constant. The dielectric constant of air is 1, and the dielectric constants of other materials differ widely. Mica, for example, has a dielectric constant of approximately 6. Let us see what significance this has. It means that if an air condenser has a certain capacitance and if mica were substituted for air, the capacitance of the condenser would increase six times. Dielectric Strength The dielectric strength of an insulating material is the minimum value of electric field intensity required to rupture it. Dielectric strength usually is expressed in kilovolts per centimeter of dielectric thickness. Heating of the dielectric leads to rapid deterioration, particularly if moisture is present, and ultimate breakdown. Most dielectrics will withstand a much higher voltage for a very brief period than voltage applied for a longer period. These effects have dictated two tests for condensers, a high flash-test voltage of very brief duration, and the application of a much lower voltage for a longer period. If too large a voltage is impressed upon a condenser its dielectric may be pierced by a spark which jumps through it. In condensers using paper, mica, or glass as the dielectric, the puncture of the dielectric by too great a voltage will prove fatal and will render the condenser useless for radio work. A condenser using oil, a wet electrolyte (borax solution), or air, will heal itself after the excessive voltage is removed. This excessive voltage is known as the "breakdown voltage." Condensers should never be operated with such a high voltage across them. The safe voltage which may be impressed across a condenser is called the "working voltage." Lower voltages may be used, but voltages higher than the working voltage JULY QUESTIONS AND CORRECT ANSWERS 1. (Q.) Find the resistance of a 2-henrv choke connected across 100 volts, 60 cycles. (A.) 753.6 ohms. 2. (Q.) Find the current that will flow through the coil. (A.) .13 amps. 3. (Q.) Find the reactance of a 50 micro-henrv coil connected across one volt at 200 kilocycles. (A.) 62.8. 4. (Q.) A reactance of 3 ohms and a 5-ohm resistor are connected in series. Find the impedance. (A.) 5.8 ohms. should never be used for more than a brief period. A condenser can be made to withstand very high voltages by increasing the thickness of the dielectric, but this is not always feasible because the capacitance of the condenser decreases as the thickness of the dielectric increases. In order to make up for the loss in capacitance it would be necessary to increase to area of the plates, thereby making a verybulky condenser. Mica condensers of this type have been used recently in some radar equipment. They were about ten times larger than ordinary "postage stamp" mica condensers and were capable of withstanding from 2.500 to 5,000 volts. The amount of voltage that a condenser can safely withstand is clearly marked on the condenser. These voltage ratings vary for different kinds of condensers— for mica condensers the values range from 500 to 5.000 volts; for paper condensers from 150 to 2.500 volts, and for electrolytics from 25 to 600 volts. Rating Condensers The size of a condenser is known as its capacitance, and the unit of capacitance is the "farad." named so in honor of the English scientist. Michael Faraday. This unit is much too large for practical work. In practical work the micro-farad (one millionth of a farad ) and the micromicrofarad also called the pica-farad (one millionth of one millionth farad) are used. The farad is the amount of capacitance present when one coulomb of energy is stored in a dielectric field under a pressure of one volt. Paper and electrolytic condensers usually are rated in micro-farads, while mica condensers are rated in micro-microfarads. The electrical symbol for the micro-farad is ^fd. and the symbol for micro-microfarad is jj./xid. The Greek letter jj. (Mu) is used to represent the word micro. Some manufacturers use the letter m in place of the Greek letter fj., so that micro-farad would be written mfd, and the micro-microfarad would be written mmfd. Sometimes the d in mfd or mmfd is omitted. Electrolytic condensers usu AUGUST 1945 19