Text 15 ammeters and voltmeters Hot Wire

When an electric current flows along a conductor the latter becomes heated, the heating effect being simple to use for meas­urement purposes. The heat produced per second in a conductor of any given resistance is proportional to the square of the current, and the conductor will, therefore, be heated up to the point at which it radiates each second all the heat which the current is producing. Its temperature rise above the surroundings will then be proportional to the square of the current flowing, and it is only necessary to measure this temperature rise in order to get a measure of the amount of the current.

The simplest way of measuring the temperature rise is by means of the linear expansion which it causes in the wire carrying the cur­rent, and-hot-wire instruments work on this principle. Unfortunately the expansion is extremely slight, being of the order of one hundred thousandth of the length for each degree centigrade, and it therefore becomes necessary to magnify the movement many times before trans­mitting it to the pointer of the instrument. This complicates what would otherwise be a very simple type of mechanism, and it also introduces errors due to the stretch of the various wires employed or to the inter­play of the magnifying mechanism. Other errors may be introduced due to the expansion of the base plate of the instrument and for these rea­sons the hot-wire instrument frequently exhibits a zero error and is usually only of second-grade accuracy.

You can easily imagine a hot-wire instrument employing what is called the double-sag method of magnifying the expansion move­ment. The current to be measured, or some definite fraction of it, passes through the expansion wire, which is made of platinum-iridium or platinum-silver, either of which alloys will stand a high temperature without oxidization. When this wire is heated and expands, its sag is taken up by a phosphor bronze wire, and the sag in this is taken up by a silk fiber F which is kept taut by a small spring. This silk fiber passes round a small pulley mounted on the instrument spindle and a very minute expansion in the hot wire causes a considerable movement of the spindle, and is shown by the pointer mounted on it.

When the instrument is required to be a voltmeter the expansion wire is connected in series with a high resistance whose value will depend upon the voltage range which is required. The current neces­sary to operate the hot wire is usually of the order 0.1 to 0.2 amp., and as the resistance of the wire itself is only 10 ohms or so, it will be seen that except on very low ranges the added resistance ab­sorbs the greater part of the voltage applied to the instrument termi­nals. Provided this added resist­ance does not vary with temper­ature, the current flowing through the expansion wire will be directly proportional to the applied voltage and the expansion will, therefore, be proportional to the square of the voltage.

When the instrument is to be an ammeter, the expansion wire can be made somewhat thicker, but if too stout, the instrument will be sluggish in action, as the wire will take an appreciable time to reach its final temperature. The sizes used in practice are therefore such as to require only a fraction of an ampere to raise them to their full temperature. Hence, when a bigger current than this is to be measured, the hot wire has to be shunted with a resistance of a lower value, and this reduces the accuracy of the instrument. As explained previous­ly, the shunt must be designed so that the current through the hot wire bears a constant ratio to the total flowing through the instrument so that the expansion obtained will be _% proportional to this current squared. The use of such a shunt is not very satisfactory, however, since the resistance of the hot wire will vary somewhat with different currents, while that of the shunt will remain constant.

It will be seen that both in ammeters and voltmeters, the expan­sion of the hot wire is proportional to the square of the quantity being measured, and although the magnification system modifies this effect somewhat, the result is always a scale very crowded at the beginning and very open at the end(шкала дуже стиснута спочатку і дуже розтягнута в кінці). This may some­times be a serious disadvantage, since small values cannot be read on a large range meter. On the other hand, the hot-wire type is particu­larly suitable for measuring alternating quantities, since the value of a current which is alternating is defined as that value which, if flow­ing continuously, would produce an equal amount of heating. Hence the hot-wire instrument, or in fact any type obeying what is called a "square law”, will read the same for alternating as for direct currents, whatever the shape of the alternating wave. This is very useful when-measurements are to be made in circuits in which the current or voltage wave-form departs considerably from the ideal sinusoidal shape.