|The Principle of a Relay
Relays in Cascade
The Principle of Dissipation of Diathesis
Intensity of Diathesis
Parallel Channels for Diathesis
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One of the first facts to note is that the path for the diathesis does not act as a transmitter of energy although it does receive a supply of energy at many points along its route. Energy is fed in at the crane-man's brain, at his nerves, at his muscles, at the contactors adjacent to the crane motors. But none of this energy passes out of the path to the casting. It is all converted into heat in making the path for the diathesis work. This path is a consumer of energy; it is not a transmitter
Moreover, each of the various quantities of energy that are fed into the path is converted into heat locally; the energy travels along the path for a short distance only, and never passes from one section into the next. Thus the energy fed in at the crane man's brain does not reach his muscles; the energy fed in at his muscles does not reach the contactors; the energy fed in at the contactors does not reach the motors.
This is because the path for the diathesis works on the principle of a relay — a principle with which engineers are quite familiar, but which I shall explain briefly for the benefit of those who are not engineers.
First let me give a definition: A relay is any device for controlling a flow of energy so constructed that the energy controlled by the device is distinct from the energy used to operate the device.
The principle of a relay is illustrated in Fig 3. This shows but one of many types. The type shown would be used to control energy in the form of an electric current. It differs only in details from the contactors adjacent to the switch motors. In this relay, the switch B is normally held open by a spring. It is closed only when the electro-magnet is energised. This attracts an armature connected to the switch and is powerful enough to overcome the force of the spring. The armature is not shown on the diagram, but it is easy to understand that the switch can be caused to close when the magnet pulls on something. So switch B is controlled by the magnet.
As relays are going to claim much of our attention, it will be convenient to have some terms with which to describe those essential parts of a relay that are common to all conceivable types. Let switch B be called the controlled element. Let the electromagnet be called the operating element. Let, further, the energy that flows through the controlled element be called the controlled energy and the energy that which flows through the operating element be called the operating energy. Let the process in which the controlled energy is consumed be called the controlled process. Lastly, let the controlled process that is at the end of the path for the diathesis be called the objective of the diathesis. In our example, movement of the casting is the objective of the diathesis. At the opposite end of the path to the objective is the source of the diathesis. Anything that can be said about this is pure theory at present, so I propose to say nothing until more of the indisputable facts have become clear.
There is no limit to the number of ways in which a relay may be constructed and there is no limit to the forms that may be taken by the operating and the controlled energy. Either may be mechanical, electric, thermal, chemical. By the rather wide definition that I have given, a water tap is a relay. So is the trigger mechanism of a gun. The trigger is the controlled element. The finger that presses on the trigger is the operating element. The energy in the cordite that is released when the trigger is pressed is the controlled energy. A bull's eye scored on the target is the objective, though it may not be achieved.
In Figure 3, the switch that constitutes the controlled element has two alternative positions. But this element need not be a switch, and to speak of alternative positions of either the controlled element or the operating element is not general enough. So I propose to speak in future of the two states in which these elements may be. When the controlled element is in one state, energy flows to the controlled process; when it is in the other state, energy does not flow. The two states may be distinguished by the position of the element, but they may also be distinguished by any other physical circumstance. The difference between the one state and the other may be a difference in an electro-static or a magnetic field; it may be a difference in chemical constitution; it may be a difference in the permeability of a membrane; it may be a difference in osmotic pressure.
Where the range of possibilities is so wide, Fig. 3 shows too much detail to be applicable to the general case of a relay. To represent this, one must show no more than a rectangle as in Fig. 4, with its inlets and outlets. There are two inlets, one for the operating energy which enters with the diathesis, the other for the controlled energy. There is one outlet which serves jointly for the controlled energy and the diathesis. There is no outlet for the operating energy as this is all converted into heat in the relay.
A characteristic feature of a relay is that it is irreversible. One can control the controlled element by means of the operating element; but one cannot control the operating element by means of the controlled element. This irreversibility can very conveniently be expressed by saying that diathesis can pass through a relay in one direction only. This provides one of the justifications for speaking of diathesis as a commodity.
Another characteristic feature is that the controlled and the operating energy need not be equal — or even of the same order of magnitude. The controlled energy may be, and often is, very great while the operating energy is very small. So it is with a gun; so it is with the starting switch of a motor car engine; so it is with a thermionic valve; so it may be with Fig. 3, where the dynamo may supply energy at the rate of thousands of kilowatts while the battery supplies energy at a rate of but a few watts. Hence one may speak of the amplification of the relay. The term means an amplification of energy, not an amplification of diathesis.
The energy that passes through the switch A is the controlled energy for relay number 1, and it is also the operating energy for relay number 2. When relays are so connected that this happens, they are said to be in cascade. This leads to the following definition: Relays are said to be in cascade when they are so connected that the operating energy of one is also the controlled energy of the next relay nearer to the source of the diathesis.
Fig. 5 shows a number of relays in cascade.
To calculate the amplification of a set of relays in cascade, one must multiply the amplifications of the component relays. The resultant amplification may be very large. If each single relay has an amplification of one thousand, the amplification of two relays in cascade is one million, and that of four in cascade is one million million. Thus a set of relays in cascade can be so designed that their energy requirements taper sharply as one proceeds from the objective towards the source of the diathesis.
To express this feature I propose to speak of the principle of tapering energy requirement. It is a principle of great importance in engineering. It is, indeed, the principle that has given man mastery over the great forces of nature.
If a quantity of diathesis is to be measured in terms of closeness of control, we have here a loss of diathesis between the brain and the casting. In general, one may say that a portion of a given supply of diathesis is always lost as this commodity passes from its source towards its objective. This observation is of enough practical importance to deserve a name. I shall call it the principle of dissipation of diathesis.
Note also that the less diathesis a machine requires for its operation, the more it requires for its design and construction.
One can speak of diathesis being more or less intense, as one can speak of greater or less intense pain, or pleasure, or surprise, or disappointment. However, there is one significant difference. The criterion by which to measure the degree of intensity of these latter experiences is, of necessity, purely subjective. But the criteria mentioned above by which to measure the degree of intensity of diathesis are objective. Though this is all to the good, I would not pretend that the criteria are precise enough for numerical statements. But then I am not sure yet that numerical statements will ever be required. Maybe it will suffice to say that the intensity of diathesis is, by an objective criterion, more or less without saying how much more or less. Should this prove insufficient, I have no doubt that a basis for numerical statements could be devised.
It may be thus in our foundry. The casting has to be placed centrally on the railway truck, for it is a large one and if any part of it overhangs the side of the truck it will be swept off by the first tunnel through which the train passes. Perhaps the craneman, from his distant cabin, cannot ensure a sufficiently central position for the casting or he dissipation of diathesis between the cabin and the casting is too great. If so, the foreman calls a labourer standing about on the floor of the foundry and says to him: "Here, Bill. Lend a hand". And Bill comes and pushes against the casting as it swings ponderously from its hook, and coaxes it into the required position. What Bill does can conveniently be described by saying that he supplies supplementary diathesis. Diagrammatically this would be represented in Fig. 1 in Chapter 10 by a tributary to the path of diathesis shown there.
In human affairs, supplementary diathesis is often fed into a path for diathesis at various places, especially if the path is a long one. We shall in due course have to consider whether supplementary diathesis is also fed into any parts of the path for diathesis that lies within living substance.
This fact provides an argument against speaking of diathesis as though it were a commodity, for this manner of speaking would suggest that the diathesis is subdivided into a number of parallel channels as water or energy could be sub-divided. I do not want to suggest this. It would rob diathesis of all its useful meaning if one were to define it in such a way that one could speak of its sub-division. It would be quite wrong to say that one diathesis passes along the circuit for control of the vertical movement of the casting, another along the circuits for control of the two horizontal movements, and three others along the circuits for control of the speed of each movement: for such a manner of speaking would ignore certain important aspects of diathesis, such as timing and co-ordination.
The timing and co-ordination of the craneman's activities with his switches is not something merely added on to his simple operation of the switches. It is an integral part of one indivisible act of control. If he did not time his operations correctly, if he did not co-ordinate them, there would be no diathesis at all. The casting would certainly not reach its specified position. To omit part of a diathesis is to destroy the whole. But when one speaks of a commodity, one usually thinks of something that is not destroyed by removal of a part. To speak of diathesis as indivisible is to speak of it rather as a process than as a commodity.