A New Instrument for Measuring X-radiation

THE fundamental principle of measurement upon which is based the instrument to be described in this paper is a very old one. It is known in physics as the “null,” “zero,” “compensation,” or “balance” method, and is generally accepted as the most satisfactory procedure for certain types of measurement. The well known Wheatstone bridge and potentiometer, used extensively for electrical measurements, are based on this principle. On the other hand, the apparatus as a whole and the component parts in their special functions have not been employed before for similar purposes, at least to the writer's knowledge. In order to bring out in a simple way the operation of the device, it is well perhaps to start with a mechanical analogy. Everyone is familiar with the weighing of objects by means of a beam balance. Let us analyze the procedure. We wish to determine the weight of, let us say, a coin by means of the simple balance shown diagrammatically in Figure 1. The first thing to do is to check the balance and if it has a “zero error,” adjust it so that the pointer P is opposite the zero mark on the dial, when the pans A and Bare empty. The coin is now placed in the pan A and, due to its weight, the pointer moves to the left. Then we add a number of weights of known magnitude to the pan B until the pointer is again at the zero mark. Under these conditions the action of the coin on the balance is counteracted exactly by the action of the weights. All other factors being equal, therefore, the weight of the coin is equal to the sum of the individual weights placed in the pan B. The essential elements in this procedure are: (1) A device which is capable of detecting the pull due to the force of gravity acting on a material body, placed in either one of two suitable receptacles; (2) means for adjusting the device to a zero position; (3) a body which, acting on the device, will cause a visible deviation from the zero position, and (4) suitable units of known weight, which, placed in opposition to the first body will restore the device to the zero position. We may now consider how a similar procedure may be employed in a different type of measurement. Let us say that we wish to measure air flow in cubic centimeters per minute. We must have again the four elements mentioned above. Air flow is capable of producing several effects, any of which may be used to actuate the detecting mechanism. Taking pressure as the effect, our detector must be capable of indicating pressure changes. Then means must be provided for setting the indicator, for instance, a pressure gauge, to any predetermined reading to be taken as zero. As the third element we must have something which produces an air current in a pipe, an air compressor. For the fourth element we shall select a suction pump which is calibrated in such a way that, knowing the speed of the pump, the discharge in cubic centimeters is known.