# Resistance coils for alternating current work (Google eBook)

Govt. print. off., 1912 - Technology & Engineering - 23 pages

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Page 23 - A theoretical discussion is given to show the conditions which must be fulfilled in the construction of resistance coils in order that the phase angle shall be small and the change of resistance with frequency negligible.
Page 8 - EFFECT OF TEMPERATURE UPON THE CAPACITY Since in many coils the phase angle is largely dependent upon the capacity, the temperature coefficient of the phase angle will be nearly that of the capacity. For a poor dielectric, such as is generally found in resistance coils, this temperature coefficient of the capacity is usually quite large. In one bifilar coil which had been cut at the middle point it was measured and found to be 1 per cent per degree.
Page 8 - It has been shown 1S that the shellac of resistance coils absorbs moisture from the air. On account of the high dielectric constant of water, one would expect that this would affect the effective inductance of the coil as a result of the change in the capacity. This was found to be the case. In one coil the effective inductance was increased algebraically by 20 per cent by baking at 110° С for 15 minutes.
Page 2 - He attributed the difficulty with the larger- valued coils to capacity between the windings. Acting on this suggestion Chaperon3 devised his well known winding. This consists in winding each layer inductively, but reversing the direction of winding of alternate layers, and thus necessitates a multiple-layer coil. This construction reduces the capacity without increasing the inductance materially. A slight improvement was claimed by Cauro...
Page 4 - ... been used. Computing by the formula of Bethenod 13 the resistance change in this case is also found to be negligible. Hence, in none of the coils which we have designed will the skin effect be appreciable. In fact, it may be said that the resistance of most commercial coils having a resistance of 1 ohm or over is not affected by the skin effect up to 3000 cycles.
Page 20 - A form of resistance proposed by Rowland " is quite satisfactory for high resistance alternating current work. It consists of fine wire wound inductively on thin sheets or cards of mica. The effective inductance of resistances of this type is small, the time constant being seldom more than 1o~e and often as small as 1o~7 second.
Page 14 - The 3o-ohm coils were wound of the same size wire as was used upon the 1oo-ohm coil just described. They were wound bifilar in the ordinary manner, the three coils being wound upon the same spool. Whether the spool is of brass or porcelain is of little importance, as practically all the capacity is between adjacent wires. Still, better results could doubtless have been...
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Page 3 - Each gives rise, also, to a change hi the effective resistance of the coil. It is the purpose of the following theoretical investigation to show how each of these three factors enters and to give an idea of the magnitude of the effects which they produce. 1. "SKIN EFFECT...
Page 8 - ... been flowing as well as upon the magnitude of the current. This drift of the effective inductance has often been observed by us, and before the cause was understood it gave us considerable trouble. The following example will show the magnitude of this for an ordinary coil. A...