To limit the current I through a discharge tube, a

ballast with a resistance R had to be connected in

series with the discharge tube T and the voltage

source U. The value of R had to be constant or, even

better, had to increase with increasing current.

According Ohm's law, the current through the circuit

was limited to its short-circuit current I = U/R. The

power to be dissipated in the resistor equalled U x I,

the product of the supply voltage and the current. At a

supply voltage of 230 volts and a desired working

voltage over the discharge tube of for instance 55

volts at a working current of 1,4 Ampere, a serial

resistor of (230-55) V/ 1,4 A = 125 Ohm would be

needed. Its power dissipation then would be (230-55)

V x 1,4 A = 245 Watt. The discharge

tube under this circumstances would

dissipate 55 V x 1,4 A = 77 W so with

this kind of limitation about three quarters

of the applied power was dissipated in a

resistor whose only purpose was to limit

the current through the discharge tube. A

good example of such a configuration is

the Original Hanau Alpinette PL36

sunlamp on display here. The resistor

right of the discharge tube was

responsible for the ignition of the lamp, as

will be explained in the next section of

this building.