CHEAP CONDENSERS

Hawke's Bay Tribune, Volume XXII, Issue 106, 19 April 1932, Page 5

CHEAP CONDENSERS

Semi-variable condensers of the compression type are widely used for a number of purposes; not tho least of the reasons for their popularity is their cheapness. As they cost but little more than a fixed condenser of the same capacity, they may often be used in place of one of these components in cases where the use of a variable capacity is likely to confer the slightest advantage. But experience shows that compresion condensers are not always used to tho best advantage, possibly because amateurs do not understand fully the principles on which they are made. All semi-variable condensers of tho type under discussion are constructed on similar lines. Essentially they consist of a series of springy metal plates, ininterleaved with mica sheets. The metal vanes are bent in such a way that when the adjusting screw is slacked off they spring apart to the maximum possible extent, and so capacity is at minimum. When the screw is turned in a clockwise direction it forces both vanes —or both sets of vanes—into closer contact, and thus capacity is progresively increased. It will at once be ob vious that such a construction must inevitably result in a fairly high minimum capacity. This point should always be remembered, and it is useless t. expect, for example, that a 0.0005 mfd. compression condenser will tune a given coil through as wide a range of wavelengths as a condenser of the air-dielec-tric type. All milkers give figures for minimum, as well as for maximum, ca-

pacities, and both should be borue in mind when making a choice. Another point to remember is that each turn of the adjusting screw docs not produce the same change in capacity, or anything like it. In fact, the majority of the capacity change takes place during the last couple of turns of the screws. Obviously, one must not conclude that something is wrong if “nothing happens” until the screw is quite near the end of its travel in the threaded guide. Although compression condensers are hardly at their best when used for tuning purposes in circuits wdiere adjustment is critical, it is often convenient to use them for such purposes. In such cases there is a possibility that on tightening the locking nut after making the adjustment, the screw, even if it is not turned accidentally, may be raised in its guide to a small extent, which may be sufficient to upset tuning. Phosphor bronze, or very hard brass, is the material used in the best condensers; others, with vanes of comparatively soft brass, may tend, afterprolonged uses, to have a higher minimum capacity than the initial value. This is due to the fact that the plates do not separate to their full extent, and matters can often bo put right temporarily by rc-bending them.