The introduction of a transducer or an electronic device based on a hitherto unused principle is nowadays something of a rarity. This is true even for the Physics Exhibition where a deliberate attempt is made to select exhibits which have new features. Usually, the novelty lies either in appropriateness for particular applications or in enhanced performance brought about by improved manufacturing techniques. Certainly this year most of the exhibits fell into one or other of these categories.

Developments of this nature can involve a great deal of effort and much disappointment before a satisfactory result is obtained. A typical example is the four quadrant position-sensitive photocell shown by the British Scientific Instrument Research Association. This is the latest of a series of photo-sensitive devices produced by the Association over several years. The device is made by cutting a p-type diffused layer of a silicon solar cell into quadrants and bringing out connections to each (see figure 1(a)). What is remarkable is that the

coordinates of a spot of light up to 0.1 in. in diameter may be determined over a dynamic range from a few micro-inches to approximately 0.05 in. (see figure 1(b)). The frequency response is also quite high (3 dB down at 50 kc/s) and another particularly useful feature is that by measuring the total current from all four quadrants the spot intensity may be determined and if necessary controlled. This is altogether a transducer of considerable promise.

It was interesting to come across another attempt to solve the problem of providing a general purpose instrument for the measurement of relative humidity. No universally acceptable solution has so far been found to meet this widespread requirement. Inexpensive methods are rather unreliable or not easy to use and accurate methods tend to be too expensive for general application. The approach of Wayne Kerr Laboratories has been to develop a transducer which uses