Unit 23 Resistance Measurement

The choice of a suitable method of measuring resistance will depend on several different factors, some of the more important being as follows. The range of resistance to be mea­sured, that is, whether low (less than 1 ohm), medium, or high (100,000 ohms and above). The required accuracy of the measurement, greater, say, than 1 per cent, or not, and the ease with which it can be made. Whether the measurements are to be made with the item tested under its normal working conditions or away from these. This latter point can be rather important when we consider that the resistance of conducting materials increases with temperature, and hence with the amount of current flowing through them, whereas the resistan­ce of insulating materials decreases with temperature rise, as well as with the absorption of moisture, where these are hygroscopic.

In any particular case the choice of method will be limited to one or two, depending on the equipment available. The various methods can be divided into two classes, laboratory, and workshop methods. This division again depends on the accuracy with which the measurement is to be made, as the same basic principles are used in many cases. "Laboratory test measurements" generally infers that the greatest preci­sion is obtained with a given method. On the other hand "workshop measurements" infers modifications to a method to give greater ease of manipulation, with direct reading if possible, and less emphasis on the accuracy obtained, pro­vided that it is within useful limits.

Exercises

I. Read fluently:

a suitable method, the required accuracy, normal working condi­tions, the method can be divided, can be rather important, increases with temperature, the amount of current flowing, the resistance of insulating materials, the absorption of moisture, with a given method, with direct reading.

II. Find in the text sentences with:

1. Nominative Absolute Construction. 2. Participles used attri­butively.

III. Find in the text English equivalents for:

отсюда, в то время как, в особом случае, с другой стороны, при условии, что, т. е.

IV. Make a plan of the text.

V. Retell the text according to the plan.

VI. Translate into English:

Практической единицей сопротивления является ом. Сопротив­ление обычных проводящих материалов, как правило, не на много больше 1000 ом. Сопротивление изоляционных материалов обычно составляет миллионы ом. Удобной практической единицей явля­ется Мегом (1.0 megohms = 1.0 million ohms = 10^е ohms). Так как электрическое оборудование включает как проводники, так и изо­ляторы, то работа оборудования будет зависеть от каждого из его составляющих (component) материалов.

VII. Supplementary reading. Text 16 ammeters and voltmeters

Moving-iron Instrument The electro-magnetic effect of a current is the one chiefly made use of for measurement purpose. Moving-iron instruments employ this effect. In principle the moving-iron instrument consists of a fixed coil of wire car­rying the current which magnetizes a small piece of soft iron mounted on the instrument spindle. In con­struction there are two varieties - the repulsion type having two pieces of iron, and the attraction type having only one.

In the attraction type of instrument illustrated in the sketches in Fig. 11 the Bobbin C carrying the wire is oblong instead of circular and has only a narrow slot shaped opening in its centre. A thin flat piece of soft iron, A_y which is mounted on the instrument spindle is sucked into this slot by magnetic attraction when the current flows, the motion being opposed, in this case, by the weight W. Either gravity or spring control can be employed on moving-iron instruments, and damp­ing is usually by means of an air dash-pot. This latter is clearly shown in both of the above figures, where it takes the form of a light piston (P. Fig. 11), fixed to the spindle in such a way that it describes an arc of a circle within a curved cylinder whose walls it nearly touches.

Moving-Coil Instruments.— In moving iron instruments the magnet­ic field is furnished by a fixed coil of wire carrying the current, and a piece of soft iron moves in this field. In the moving coil instru­ments now to be considered, this arrangement is reversed; the field is provided by a fixed permanent magnet, whilst a coil carrying the current moves within this constant field Fig. 12. shows the general arrangement of a modern moving-coil permanent-magnet instrument. It will be seen that the instrument consists of a permanent magnet M, having two pole pieces, P. In between these pieces a circular iron core I is fixed by means of the bracket B, the latter being made of some non-magnetic material such as brass. This leaves a small annular air-gap in which the coil F is free to turn. This coil consists of a light aluminium framework carrying a number or turns of fine silk-insulated copper wire.

When the current flows round this coil the latter tends to turn in the magnetic field provided by the permanent magnet, and its motion is opposed by the spiral hair spring C, of phosphor bronze, which also serves to .lead the current into the coil. Usually there is a second control spring fitted at the other end of the instru­ment and wound in the opposite direc­tion so as to neutralize temperature variations in the spring. This will then serve also to lead the current out of the moving coil, but in some cases a flexible metallic ligament serves this purpose and exerts no appreciable restriction on the movements of the coil