790
��Popular Science Monthly
��brated sufficiently to act as a transmitter when connected in on the circuit as shown. Microphones may be made very sensitive, and, when combined with the sound- amplifying contrivance here described, should make exceedingly faint sounds audible.
The induction coil shown in the micro- phone circuit is not strictly necessary, but
���contact
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Bottery '
A microphone made of a cigar box and its wiring diagram to the induction coil
��it will improve the operation of circuits. The primary (big-wire) side goes in the battery circuit, and the secondary (fine wire) side in that of the receiver. An in- duction coil of suitable size may be pur- chased from any telephone company.
��Switchboard Constructed for Use in the Laboratory
THE illustration shows a simple com- bination of switches that can be used to flow a current of electricity in different strengths for making tests in a laboratory. The switches can be con- nected with the ordinary commercial line carrying 110 volts. Referring to the
���A combination of switches on a base for directing electric current for a laboratory
illustration, when 1, 4 and 5 switches are in contact, a straight current flows at the
��terminals. Switches 1 and 4 cuts in a series through the coil, and 3, 4 and 5 a series through the light. The switch 2 cuts the light into the circuit and 5 cuts the coil into the circuit.
A common arc light coil is connected to the terminals at the point marked coil and a 110-volt lamp at the place marked light.— T. I. Dekle.
��Alternating Current Charging With- out Rectifier
IT is not absolutely necessary to have a rectifier with an alternating current for charging storage cells. Imagine that we have a 6-volt storage battery in need of charging and that the house mains pro- vide current at 110 volts A. C. We hook in a bell transformer stepping down the voltage to nine volts as shown by the sketch, and then connect six ordinary dry- cells — new ones — in the secondary cir- cuit, the dry cells being in series with the storage battery to be charged. It is es-
��Tronsformer
���6 storoge bollera Hookup for dry cells with transformer to rec- tify alternating current for battery charging
sential to connect the carbon pole of the dry cell battery to the positive pole of the storage battery.
Now to show that the battery is actually being charged: Let us consider conditions when the direction of flow of the A. C. is the same as the direction of flow of the dry cells — for convenience we will call this direction positive. Our transformer gives us 9 volts in a positive direction; the dry cells another 9 positive, while the storage battery gives us nega- tive 6. The algebraic sum or resultant voltage is plus 12.
Consider now the other or negative alternation. Our dry cells give us plus 9; our transformer minus 9 and our stor- age battery minus 6. Resultant, minus 6. In other words, on the positive or charging alternation, we have 12 volts acting to force current through the bat- tery, while on the negative alternation we
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