# Volltext: Optic Projection: Principles, Installation and Use of the Magic Lantern, Projection Microscope, Reflecting Lantern, Moving Picture Machine

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RHEOSTATS AND OTHER BALLAST
[Ch. XIII
case there are two coils (fig. 291). If an alternating current supply
is connected with the primary coil an alternating current can be
drawn from the secondary coil.
The voltage and amperage
which can be drawn from the
secondary coil will depend upon
the electric supply and upon the
relative number of turns of wire
in the primary" and in the second¬
ary" coils. If the number of turns
is the same in both, then the
voltage and amperage remain
practically the same as if the
coils were not present. In other
Fig. 290. Use of a Special Trans¬
former with an Arc Lamp.
6' Dynamo.
=*= Alternating current circuit.
T Transformer.
A Arc lamp.
The primary of the transformer is
connected to the dynamo while the
secondary is connected to the arc words the circuit is in everv way
lamr
ip.
The transformer has sufficient “re¬
actance" to serve as a ballast for the
arc as well as to act as a step-down
transformer.
almost as if the wire were contin¬
uous. If the transformer were
perfect the voltage and amperage
would be exactly" the same as if it
were not present. In practice they* are a little less, but a good
transformer gives an efficiency" of 95% to 98%.
If the secondary coil has a different number of turns from the
primary coil then the voltage will vary directly as the ratio of the
number of turns in the two coils, and the amperage will vary
inversely" as that ratio. That is, assuming that there is no loss in
the transformer, the watts delivered will remain constant as the
product of volts x amperes remains the same.
For example, suppose the secondary coil has J^ th as many turns
as the primary" coil, then the number of volts across the secondary
will be }4th the number across the primary" and the number of
amperes delivered by" the secondary" will be four times the number
drawn by" the primary". If now the primary" is connected to a 220
volt line there will be a potential difference of one-fourth that
number or 55 volts across the terminals of the secondary coil.
Suppose the secondary" coil supplies 60 amperes, as might be the
case with an arc lamp, then the primary" coil would draw one-fourth
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