Text 18

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Auto-transformers

The transformer effect can also be obtained with a single tapped winding instead of separate primary and secondary windings. The arrangement is called an auto-transformer. If the primary winding represents the whole coil, the secondary voltage will be substantially the same proportion of the applied voltage as the proportion between the turns up to the secon­dary tapping and the total number of turns. By interchanging the voltage so that the supply is connected to the smaller number of turns, a voltage larger than the supply voltage ap­pears across the whole coil. The auto-transformer can thus be used to obtain a higher or lower voltage than the supply, as in the case of the convential transformer with two separate windings.

In practice, the use of auto-transformers is limited to fairly small voltage ratios, one reason being that if a break occurs anywhere in the secondary section of the winding, the primary voltage is applied to the apparatus connected to the secondary. With a high primary voltage this would give dangerous con­ditions. The chief uses of auto-transformers, are in a. c. vol­tage regulation and for infrequent service such as the low-voltage starting of induction motors.

Answer the questions:

1. How can the transformer effect be obtained?

2. How is the arrangement called?

3. Where are the chief uses of auto-transformers?

Text 19

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Precision d.C. Amplifier.

This is a precision instrument designed to measure small direct currents, as little as 10^-13 amps, voltage and small increments of charge. It will also provide a stabilized 105 volts d.c. h.t. supply to polarise an associated ion chamber.

Silicon transistors have been used wherever possible in the interests of reliabibility and stable performance. Indication is provided by a front panel meter and provision is made to operate an external meter or recorder.

Construction

The first stage of the amplifier uses an electrometer valve and this, together with the associated range switch and high value resistors, is housed in a robust sealed box, which is protected from the effects of moisture by a de­siccator. The remaining amplifier stages are mounted on a printed circuit board, connections to which are effected by two quick-release multi-plugs and sockets. In the case of breakdown, it is thus possible to replace either or both the electrometer box and the printed circuit board easily and quickly.

The cabinet is made of sheet steel finished in hammered grey enamel and will withstand the sort of environment likely to be found in laboratories and schools.

Answer the questions:

1. What does the first stage of the amplifier use?

2. How is the box protected from the effect of moisture?

3. What is possible to do in the case of breakdown?

Text 20

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