"Ken Layton" wrote in message
...
On Friday, October 23, 2015 at 9:07:18 PM UTC-7, wrote:
Cylindrical. Axial leads.

Ordered caps for a speaker crossover because the leads had literally been
shaken out of the originals. But I had no luck finding those bulbs. I
would rather not jump it out. I guess I could and just put a beefier horn
in it.

It's called a "barreter". It's just a lamp used as a current limiter.

The earliest reference I could find to baretter was from the pioneering days
of wireless - apparently it was used as a detector.

The type I remember from the days of AC/DC TVs and radios was an iron wire
filament enclosed in a hydrogen filled envelope.

The series chain of heaters had a very low cold resistance, so the baretter
was included in series with any dropper resistor to reduce the surge current
when switching on from cold.

In later equipment, NTC themistors became the norm, they reduce in
resistance as current through them causes them to heat up, so they
compensate for the low cold resistance of heaters.

The Polyfuse is a PTC thermistor; the resistance increases with temperature,
at room temperature the resistance should be insignificant in the
application, too much current causes heating and the Polyfuse has a sharp
knee, so it cuts off current to the load till it cools again.

The thermistor types have significant recovery time - the PTC thermistor
used to deliver a decaying burst of AC to the degauss coil in a CRT display
had about 6 minute recovery time.