this particular detecting element illustrates why the plant engineer is slow to take up new ideas , for at first sight to introduce microwave generating and detecting equipment into a power station fills the plant engineer with horror . it is only when the equipment can be made rugged and utterly reliable that he will consider using it at all . nowadays engineers tend to use detecting elements which give an electrical output . the reason for this is that it is a matter of extreme simplicity to amplify the signal to any degree which is necessary . moreover , it is very easy to transmit the signal from one part of the plant to another without serious loss . detectors having an electrical output are therefore of growing importance at the present time , although in many cases a mechanical output is still quite satisfactory . pneumatic devices . it is surprising that so little use is made of pneumatic devices for measurement and control of small distances . work which has been done in the British scientific instrument research association has shown that pneumatic gauging is an almost ideal way of deciding if a sliver of a semiconducting material is of the right dimensions for manufacturing a transistor . pneumatic bearings also have a considerable application which has not been developed outside gyroscopes : for example , a patent has recently been taken out covering the use of a pneumatic bearing for a glass polishing head . passing on to optical detecting elements , which are now beginning to receive the attention they deserve as a result of the application of electronic devices to replace the human eye , these are coming more and more into favour as on line instruments . the nondispersive infra-red spectrometer is a good example , while the automatic saccharimeter developed at the national physical laboratory has a good future , but it is when one comes to consider the more sophisticated optical electronic instruments that one finds the numerous advantages of utilising the visible and near visible portions of the electromagnetic spectrometer . the Hilger &amp; Watts automatic spectrographs are now well known and are in constant use in the iron and steel industries , while optical methods are beginning to be used in the guidance systems of many of our guided missiles . the importance of electrical detecting elements has already been stressed . of these , the piezo electric effect is the best known and most used , apart from the obvious conventional cases of the thermocouple and the resistance thermometer . the piezo electric effect can be used to launch ultrasonic waves in a liquid or in a slurry , and the resulting phenomena are only now being investigated on an industrial basis . nucleonic instruments tend to be largely electronic devices . the detecting element itself generally uses a nucleonic phenomenon , but the remainder of the apparatus is electronic . the reason for this is that nucleonic detection usually takes place in a very short space of time and , of course , the big advantage of electronics is its speed of response . the preceding paragraph gives some general reflections on detecting elements . a book could easily be written on this subject without exhausting the possibilities . mechanical , hydraulic , pneumatic and electrical amplifiers are all in use in automatic control systems , and these represent the second class of component into which the system can be resolved . mechanical amplifiers are exemplified by levers , while hydraulic amplifiers are exemplified by transference of a pressure or a flow from a wide tube to a narrow one . pneumatic amplifiers operate in much the same way as hydraulic amplifiers , but offer greater diversity in their application . it should be noted that in none of these cases is there any real gain in the energy of the signal . for example , a lever increases the movement which is available to the observer , but it does so at the expense of the effort which is available at the end of the lever . the same is true of the hydraulic and pneumatic amplifiers which have been mentioned . nevertheless , amplifiers have been made which correspond exactly to the electronic amplifier : the signal strength is actually increased at the expense of a reservoir of gas or liquid . electronic amplifiers are essentially devices which transform part of the direct current available from the power supply to signal current , which can then be used to perform an operation . the third element of a control system is the transmission itself . at the present time the transmission is very often done hydraulically or pneumatically , but electrical systems are gradually coming into use , subject , of course , to the stringent conditions of intrinsic safety . where long runs are required , electrical transmission is obviously to be preferred . the use of computers . the next item in the control system has gained considerable notoriety and is sometimes thought by the uninitiated to be the principal component . it is the data logger or computer . computers were originally manufactured in analogue form to solve certain complex differential equations and , in the first instance , they were mechanically operated . thereafter , electronic computers came in , operated digitally , and since then there has been competition between the analogue computer and the digital computer . to a certain limited extent the analogue computers are very useful for the examination of plant characteristics , and such computers can be used to advantage when a new plant is being set up which is to be automatically controlled throughout . on the other hand , where extreme accuracy is required the digital computer is the only one to use and electronic digital computers are employed to advantage in performing difficult calculations in optics , in stresses and strains in aircraft , and in a multitude of other problems . so far as process control is concerned , after the preliminary investigation has been carried out by means of an analogue computer , the equipment to be used on the plant should be as simple as possible and should comprise a detecting element , a data logger and a controller , the third being connected , of course , by transmission lines . the data logger is usually called upon to perform one or two simple operations and , as such , it is not worth while using a general purpose electronic digital computer to do the job . it is therefore the writer &apos;s opinion that general purpose computers have no long term significance so far as process or machine tool control is concerned . rather the data loggers and computers which will be used in these circumstances will be small black boxes , designed to do specific jobs . it will be seen that the problem of the computer is in no way related to the problem of the detecting element . when we are concerned with the right kind of detecting element to use for a particular purpose , this takes us into the background of science to examine all the various phenomena and decide on the right device . on the other hand , so far as data loggers and computers are concerned it is a matter of straightforward engineering , for the circuits and devices to be used to perform the various specific tasks are all well understood . there is only one case where this may not be completely true , and that is where extreme speed is required in the computer , but this occurs so seldom in process control or machine tool control that it is hardly worth considering . the controller . the last element in the automatic control system is the controller itself . this has to be a mechanical device since it is applied to the line and changes certain parameters therein . controllers nowadays are usually described as three term , meaning that they have a proportional control , a control which is determined by the rate of change of the signal , and a control which is determined by the integral of the signal . three term controllers require careful setting up , and to make the best use of them an exhaustive analysis of the plant is necessary , but there is no doubt that three term control is essential in most cases if the plant is to operate at optimum efficiency . the necessity for the three term controller is to be found in the mode of variation of any particular parameter . for example , if we are concerned with temperature measurement and the temperature should suddenly shoot up , then some degree of anticipation is given to the controller by means of the rate of change of the signal . on the other hand should the parameter vary slightly between fairly wide limits over a long period of time , then it is very difficult to maintain it at the desired value unless an integral of the signal is used in the control system . no doubt more complex controllers could be manufactured and may be used in the future , but in the meantime the rate of change , the signal itself , and the integral of the signal give sufficient control . the instrumentation of reactors and conventional electrical power units . whether coal or uranium is used as the fuel , the power unit must always contain certain basic automatic controls . at the present moment the output of any station is in the form of electricity , which usually comes from a turbogenerator . certain conventional instrumental controls are necessary at this end , but with the increasing use of reactors as power producing units , a completely new set of problems has been posed to the industrial instrument manufacturer . apart from the measurement of such novel parameters as the neutron flux in the reactor , the control of temperature has become of major importance . an elegant solution to this problem is not yet in sight , but reactors are able to operate using a very large number of detecting elements which measure temperature . one of the outstanding problems of reactor instrumentation is the measurement of flux of intermediate energy neutrons . the British scientific instrument research association began an investigation of this problem some three years ago and the results so far achieved are promising . as might have been expected , on the way towards the solution of the set problem ( the measurement of flux of intermediate energy neutrons ) many other problems have been brought to the notice of the association and have been solved . it is probable that the work at present in progress on new types of phosphors will result in a new set of instruments becoming available to the reactor engineer . if this is so , the economics of reactor manufacture and operation will need to be completely revised . power units in industry . almost every big factory in the country produces a large amount of steam , which is then used for many purposes . these power units are all very similar and one would expect that their instrumentation would be well known and well defined . this is not the case , however . apart from the measurement and control of fuel , steam pressure , water , and alkalinity or acidity , there are many other factors which must be measured in an economic power plant . the air to fuel ratio , the carbon monoxide content in the flue gases , and the smoke issuing from the chimney must all be controlled , and almost every factory has its individual system . there is certainly room here for a large amount of standardisation and , among smaller firms , for education in the value of adequate instrumentation . future trends . it is a difficult matter at this juncture to specify the future trends of instrumentation among our basic industries , and so it may be well to deal , first of all , with those matters which are well defined . there is no doubt that large chemical plants could use to advantage on line instruments to perform simple chemical analyses , but in many cases progress is at a standstill because it is very difficult to imagine a detecting element which can be successfully applied to a plant . another trend which has been mentioned above is towards the small special purpose computer . the general purpose machine usually contains much more than is necessary to perform its operation on the plant , and it is only by cutting out these unnecessary devices that the computer can be made an economic proposition . this trend is fairly certain . the other trend which appears to be well established is towards the detecting element having an electrical output . 