In the original meter bridge the wire was one meter long, whence its
name, and was stretched straight. In more recent examples the wire
varies in length and in one form is bent into a circle or spiral, so as
to make the instrument more compact.
The contact is not a sliding one, but is adjusted by trial. The contact
piece is slid along, but not touching the wire, and from time to time is
pressed down against the wire. This prevents wear of the wire. The wire
may be made of platinum or of platinum-iridium alloy. The latter is
very hard and not easily worn out.
Sometimes, as shown in the cut, three parallel wires are stretched along
the baseboard of the instrument, and arranged so that a single wire, two
wires or three wires in series can be used for the proportional sides of
the bridge, thus making it a two-meter or three-meter bridge as desired.
On the other hand some are made of restricted length, as a half or
quarter meter only.
Fig. 240. METER BRIDGE.
In the cut J K is the wire, traversed by the contact key. By moving the
contact C back and forth in the slot it can be brought over any of the
three divisions of the wire. H is the handle for depressing the key. S
is a flat spring, carrying the contact piece and holding it up from the
wires, except when pressed downwards. As shown in the cut, it is in use
for calibrating a voltmeter V, by Poggendorff's method, G being the
galvanometer and r1 and r2 being resistances.
Synonyms--Slide Bridge--Slide Balance.
Meter Candle.
A unit of illuminating power; the light given by one standard candle at
a distance of one meter. The ordinary units of illuminating power are
altogether relative; this one is definite.