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Show o Radio Club Notes Conducted by G. S. Morris Lesson 2, Part 2. ..Capacity, (Condensers) (Con-densers) As previously indicated, a condenser con-denser in a circuit increases the length of time required for a given potential to reach its maximum. maxi-mum. In addition to this a condenser, con-denser, by reason of its storing properties, will exert a counter EMF which action is used in circuits, cir-cuits, particularly radio, to off set the undesirable property of inductance in-ductance when a change in frequency fre-quency is desired. The selection of a condenser is governed by the required length of time it takes the condenser to reach capacity of storage and the -frequency employed. em-ployed. In other words it is desired to have in a given frequency, a size of condenser which will reach the charging source EMF at the same time the charging source reaches its peak and starts on the "voltage drop" portion of alternation. alterna-tion. As the current involved in radio frequency (radio) is small and the frequency so great it necessarily requires a condenser used in a radio frequency circuit to be of very small comparative capacity and, as the current is small, the dielectric may likewise be of Comparatively Com-paratively thinner material. In addition to this, the unit of capacity capa-city which is the "farad" would prove too great to utilize in calculations calcu-lations so this unit is divided into portions of a farad called "microfarads" "micro-farads" and "micro-micro-farads" the latter term now called "picofarad". "pico-farad". Thus a condenser whose capacity is rated as BOO MMF is really a .0005 microfarad; likewise a condenser rated as a 5000MMF is in reality a .005MF. This simple rule will prove valuable as most condenser ratings at the present time are indicated by MMF. In a previous diiscussion we found why it was necessary to alter the number of turns of a coil to change the inductance value for a variation in frequency; also we found that the proper value might fall at a point of one-half turn or other fraction of the coil winding. To offset this, which you remember remem-ber is the property tending to prolong pro-long the original flow of current, we may utilize the counter EMF property of the condenser, which will exert a pressure opposite to that of inductance and cancel it out, making the value of inductance induct-ance and capacity close to zero, which condition is much desired. It is therefore indicated that for a change in frequency we may vary the capacity of the condenser by moving the rotor plates in respect to the stator, and exert a pressure pres-sure found necessary for that particular par-ticular frequency. Therefore, even if the proper value of inductance induct-ance falls at some point within one turn of wire we may, in effect, reach that value by condenser compensation. The greater the frequency, the less value of inductance in-ductance required therefor and the less capacity required as a counter EMF to that inductance. In other words, at high frequencies frequen-cies and feeble current, we do not want a large value of inductance or capacity. The action outlined above partically eliminated the necessity of "taps" on coils except such taps as are placed at longer intervals, the value between tap intervals being taken care of by the condenser action. I Condensers of comparatively i large capacities and employed in comparatively large potentials will i manifest other properties of dis-: dis-: charge, the amount of which is dependent on the dielectric and I EMF. For example,' a condenser j charged with high voltage DC will hold that charge for a length of ' time dependent on the dielectric. ! As the condenser terminals are i slowly brought together a dis-1 dis-1 charge will take place, the first of ' which is called the "free" charge, j The second or third discharge is termed the "residual" charge. As j this residual charge is manifested by he:it in the dielectric and in u loss of energy it is therefore required, re-quired, to prevent residual charges, to select a dielectric with high dielectric constant or insulating qualities. Dielectrics possessing high insulating constant are glass, mica, air, transformer oil, bake-lite, bake-lite, hard rubber, etc. Each of the constants mentioned of course varies with its respective physical properties. For example, photo glass, such as used for photographs, photo-graphs, has a higher dielectric constant than window glass. |
