ELECTRICAL MEASURING UNITS AND INSTRUMENTS

Any instrument which measures electrical values is called a meter .An ammeter measures the current in amperes. The unit is named after Andre Marie Ampere a French scientist, who discovered a great number of facts about electricity over a hundred years ago. The abbreviation for the ampere is amp. A voltmeter measures the voltage and the potential difference in volts. The volt is named alter Alessandro Volta, an Italian scientist.

The current in a conductor is determined by two things, the voltage across the conductor and the resistance of the conductor. Every material object offers some resistance to the flow of an electron current through it. Good conductors like the metals, copper, silver and aluminium offer very little resistance, while nonconductors such as glass, wood and paper offer a very high resistance. The unit by which resistance is measured is called the ohm. The resistance in practice is measured with the ohmmeter. A wattmeter measures electrical power in watts.

Answer the questions:

1. What is called a meter?

2. What does an ammeter measure?

3. What is the current determined by?

4. What can you say about non conductors?

Для спеціальності КВП

TEXT 16

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PREFERRED VALUE RESISTANCE BOX

This Resistance Box consists of four separate switches which will select resistances in the 10% preferred value range. Dial 1 resistors from 100 ohms to 820 ohms. Dial 2 resistors from 1 Kohm to 8.2 Kohms. Dial 3 resistors from 10 Kohms. Dial 4 resistors from 100 ohms to 820 Kohms.

The individual resistors are high stability carbon types with an accuracy of ± 1%.

Each dial is individually connected and screened from the other three dials in the box so that separate dials can be used in different boxes of the same circuit without interconnection. Connection is by co-axial plugs. The whole is contained in a metal case.

Rating: 1 watt.

Answer the questions:

1. What does the Resistance Box consist of?

2. What are the individual resistors?

3. How is each dial connected and screened?

TEXT 17

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DUAL TRANSISTOR POWER SUPPLY TYPE 423

The Type 423 Power Supply is manufactured on an open chassis and is intended to be incorporated into professional equipment but it can equally well be used on the laboratory bench.

The unit provides two completely independent and isolated supplies and the output can easily be adjusted by the purchaser. The voltages provided cover those commonly used by transistor circuit designers.

SPECIFICATION

(the following specification applies to both supplies)

Output Voltages: 10-12v,15-18v, and 24v.

Output Current: 0-1Amp.

Ripple and noise:(at 1 Amp) Less than 5 mV pk.

Supply Voltage: 100/220 v

Dimensions:

Height 5.0 ins.(12.70 cms)

Width 6.0 ins. (15.24 cms)

Weight 8.0 lbs.(3.6 kg)

Answer the questions:

1. How can the Type 423 Power Supply be used?

2. What does the unit provide?

3. What is the voltage supplied?

4. What are the dimensions?

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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