=435.--The electric condenser= (see Art. 231) is a very useful device in a.-c. circuits; _e.g._, in telephone sets used in cities, a condenser is used in the ringing circuit, as shown in Fig. 436. Alternating current is required to ring such a bell and a condenser permits an a.-c. current to act through it, although it entirely prevents the flow of a direct current. This peculiar action will now be explained. =436. The action of a condenser= in an alternating current circuit may be illustrated by the following experiment. Connect twelve, 1 m.f. (microfarad) condensers, in parallel, and then attach them to a 110 volt a.-c. line so that an incandescent lamp is in circuit as shown in Fig. 437. The lamp will be found to glow brightly, although there is no electrical connection between the two sets of condenser plates. If the same arrangement is connected to a 110 volt direct current circuit, the lamp does not glow because it is really an open circuit. The lamp glows on an a.-c. circuit because, although no electricity flows _through_ the condenser, it does flow _into and out of_ the condenser, surging back and forth through the lamp with sufficient intensity to cause it to glow brightly. When the a.-c. current moves one way in the circuit, one set of plates of the condensers becomes charged positively, the other, negatively. When the a.-c. current reverses, the charges on the condenser plates reverse. In the ordinary lighting circuit 120 reversals take place each second, so that electricity rapidly flows into and out of the condensers. On removing one condenser after another from the circuit, the lamp is found to glow less and less, till when but one condenser is left, no glowing is observed, since one small condenser does not have sufficient _capacity_. [Illustration: FIG. 437.--Twelve condensers in circuit with an incandescent lamp.] The unit of capacity is the _Farad_. Capacity is defined as the quantity of electricity per second that flows into a condenser when the voltage at the terminals changes at the rate of one volt per second. If a change of one volt per second causes one coulomb to flow per second, that is, a current of one ampere, the capacity is one _farad_. The condensers used in the above experiment have a capacity of one microfarad, or one millionth of a farad. A condenser, on account of its capacity, causes an a.-c. current to _lead_ the voltage, that is the current reaches its maximum value before the voltage does. In this respect a condenser has an effect opposite to that of the self-induction of a choke coil (the latter causing the current to "lag"). (See Fig. 435.)