to form two strands, and these are wound simultaneously on the core, thus forming two spirals of equal number of turns.  The current in traversing the entire coil must flow through one spiral in one direction with relation to the core, and in the opposite direction in the other spiral, thereby nullifying the inductive effects of one spiral by those of the other.  This is called a non-inductive winding and is in reality an example of differential winding.

Lamp Filament. An excellent type of non-inductive resistance is the ordinary carbon-filament incandescent lamp.  This is used largely in the circuits of batteries, generators, and other sources of supply to prevent overload in case of short circuits on the line.  These are cheap, durable, have large current-carrying capacities, and are not likely to set things afire when overheated.  An additional advantage incident to their use for this purpose is that an overload on a circuit in which they are placed is visibly indicated by the glowing of the lamp.

[Illustration:  Fig. 117.  Mica Card Resistance]

[Illustration:  Fig. 118.  Iron-Wire Ballast]

Obviously, the carbon-filament incandescent lamp, when used as a resistance, has, on account of the negative temperature coefficient of carbon, the property of presenting the highest resistance to the circuit when carrying no current, and of presenting a lower and lower resistance as the current and consequent heating increases.  For some conditions of practice this is not to be desired, and the opposite characteristic of presenting low resistance to small currents and comparatively high resistance to large currents would best meet the conditions of practice.

Iron-Wire Ballast. Claude D. Enochs took advantage of the very high positive temperature coefficient of iron to produce a resistance device having these characteristics.  His arrangement possesses the compactness of the carbon-filament lamp and is shown in Fig. 118.  The resistance element proper is an iron wire, wound on a central stem of glass, and this is included in an exhausted bulb so as to avoid oxidation.  Such a resistance is comparatively low when cold, but when traversed by currents sufficient to heat it considerably will offer a very large increase of resistance to oppose the further increase of current.  In a sense, it is a self-adjusting resistance, tending towards the equalization of the flow of current in the circuit in which it is placed.

CHAPTER XII

CONDENSERS