## Other Instruments

Temperatures can also be measured in the following ways:

1 Two sheets of different metals are bonded together by rolling, to form a bimetal combination which bends when warmed because the metals expand differently. The bending controls a pointer which indicates the temperature on a suitably calibrated scale. This is the usual basis for an instrument called a thermograph which records temperatures on moving graph paper.

2 The electrical resistance of a wire increases with higher temperatures and can be measured by means of a battery and an electric current meter.

3 Thermal expansion of alcohol in a closed, curved tube of metal causes the tube to straighten and thus to move a pointer along a temperature scale.

4 Heat applied to the junction of a pair of different metals, A and B, makes an electric current flow from one to the other, i.e. there is a voltage difference between the metals which depends on the junction's temperature. So a loop consisting of two different wires, joined at the ends to make two junctions, creates a voltage which depends on the difference between the temperatures of the junctions. This arrangement is called a thermocouple and many junctions in series (i.e. a zig-zag of ABABAB, etc) comprise a thermopile which provides a voltage large enough to be measured easily. Alternate junctions of the thermopile are exposed to the temperature to be measured T (°C), with the others kept at a known temperature, usually that of melting ice (0°C) so that the voltage is proportional to T.

5 A portable radiation meter carried by an aircraft or satellite permits rapid measurement of the temperatures (Note 2.C) of surface or cloud top beneath, from the amount and dominant wavelength of the radiation emitted (Notes 2.B and 2.C).

In each case, the thermometer measures only the temperature of the sensing element. In a mercury-in-glass thermometer, the reading shows the temperature of the mercury in the bulb at the end, which is not necessarily the temperature of its surroundings. The reading is increased if the thermometer is exposed to the Sun, even though the air around is unaltered. A wet thermometer underestimates the air temperature because evaporation at the surface of the bulb cools the mercury. Therefore accurate measurements of air temperature require shading of a dry thermometer bulb and a brisk airflow past it. Stronger ventilation of the bulb gives a reading nearer to the air temperature, one less affected by radiation.

Figure 3.2 Six's thermometer, combining a minimum thermometer (on the left) and a maximum thermometer on the right. In each column there is a metal index pushed up by mercury, which itself is moved by the expansion of alcohol in the upper part of the left column. The bottom of each index shows the respective extreme temperature, whilst the current temperature is shown in each side by the mercury meniscus.

Figure 3.2 Six's thermometer, combining a minimum thermometer (on the left) and a maximum thermometer on the right. In each column there is a metal index pushed up by mercury, which itself is moved by the expansion of alcohol in the upper part of the left column. The bottom of each index shows the respective extreme temperature, whilst the current temperature is shown in each side by the mercury meniscus.

The thermometer must not be held too close to the observer or for too long, or with fingers near the bulb. Thermometers should be read to the nearest tenth of a degree and as rapidly as is consistent with accuracy.