What is meaning of forms in "they are even used as coil forms for inductors?"

Question

From the resistor introduction in The Art of Electronics third edition:

Resistors are truly ubiquitous. There are almost as many types as there are applications. Resistors are used in amplifiers as loads for active devices, in bias networks, and as feedback elements. In combination with capacitors they establish time constants and act as filters. They are used to set operating currents and signal levels. Resistors are used in power circuits to reduce voltages by dissipating power, to measure currents, and to discharge capacitors after power is removed. They are used in precision circuits to establish currents, to provide accurate voltage ratios, and to set precise gain values. In logic circuits they act as bus and line terminators and as “pullup” and “pull-down” resistors. In high-voltage circuits they are used to measure voltages and to equalize leakage currents among diodes or capacitors connected in series. In radiofrequency (RF) circuits they set the bandwidth of resonant circuits, and they are even used as coil forms for inductors.

In the last line of above paragraph, I don't know how to understand forms.

Which meaning of forms is used here, or is there another meaning?

1. When manufacturing inductive coils, resistors are used as molds, serving as the supporting structure for the inductor coil and helping to maintain the shape of the coil.

2. To be the thing, or one of the things, that is part of something else, often having a particular use, namely resistors as part of inductors.

Maybe the second meaning is wrong, especially "resistors as part of inductors", because inductors are only made from coils. For first meaning, molds can be replace with other the shape of the coil, so I'm confused.

Answer 1

Here, from 1957, is an example of a carbon ('composition' type probably) resistor used as an inexpensive and easy-to-source coil form (or 'former', I'm not sure there's a generally accepted definition of 'form' vs. 'former' in this context).

The resistor serves no electrical purpose- it's just a stable non-magnetic bobbin on which to wind the coil (also providing firmly attached leads), and the resistance value is chosen to be high enough that the 'Q' of the coil is not significantly impacted. In this case, the outer resistor body would be phenolic which has excellent electrical properties.

From this vintage Allen-Bradley catalog we have a depiction of the internal construction- the element is just bulk material inside a "hot-molded" insulating shell.

Modern film resistors may not be as good for this purpose- the end caps and leads tend to be ferromagnetic (because iron is cheaper than copper) and the bodies are covered by a thin layer of lacquer. The spiral element being so close to the coil turns might also have some ill effects since it is relatively low resistivity (there is a long spiral rather than a bulk resistance) and may even be magnetic in the case of metal film resistors.

Answer 2

For inductors, the word "form" is used for the thing you wind the coil on. It is what gives form to the coil while winding it.

• In some inductors, the form is removed after winding. These are usually coils made of stiff wire that will keep its shape without the form.
• In some inductors, the form remains in the inductor after winding. These inductors usually have fine wire or are very small - the wire can't hold the coil shape by itself.
• In some inductors, the form is part of the inductor. The form in such cases may be of metal or pressed ferrite particles. The metal or ferrite core increases the inductance, making it possible to get a higher inductance in a smaller volume.

Hobbyists used to wind their own inductors rather than order a specific value from a manufacturer. In such hand made parts, it was common to wind the inductor around the body of a resistor. If the wire was stiff, you could later remove the resistor. If not, the carbon composition contruction of the resistor body meant that it wouldn't have any noticeable effect on the inductance of the coil.

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In really olden days, people made simple AM radios using large air-core coils. They would wind the coil around a cylinder shaped box of oatmeal, then remove the box when the coil was complete.

The oatmeal box was the form.

Answer 3

To be entirely accurate, when a resistor is used as a form for an inductor, what you get (if the inductor is connected to the resistor at both ends) is a parallel LR circuit.

A long time ago I built a kit audio amplifier in which there was such a circuit in the output, to help with stability. You had to wind about 10 turns of wire onto a fairly large power resistor, then solder the ends of the wire to the resistor legs. This was an example of a resistor providing the form for an inductor.