General Remarks.
A high vacuum gives greater penetrative power than a low vacuum. Where the operator has not an adjustable tube it is imperative that he have at least two tubes, one high and one low. It is the contrasts which render the X-ray practical, and these contrasts are largely governed by the vacuum. In locating a metallic substance in the human body a high vacuum tube would be needed, that the bones and dense tissue be rendered more transparent. On the other hand, to make a radiograph of the bones, a lower vacuum is necessary in order to get a contrast between the bones and the tissues. In general, a high vacuum is best for fluoroscope work and a low vacuum for making pictures on a photographic plate. Short exposures in radiography are obtained by powerful rays and consequently by coils operating at considerable energy. In extended examinations or where a subject is under the X-rays for more than a minute or so, a screen should be interposed between the subject and the tube to avoid the burning effect which is often noticeable. This screen consists of a piece of cardboard well covered with gold leaf, and should be grounded—that is, a connection be run from the gold surface to a water-pipe or other ground connection. Sheet lead is an efficient screen to the rays, and, if desired, a lead screen can be made, partially enclosing the apparatus, to protect the operator. But it must be large enough and far enough distant from the coil and tube to avoid any possibility of leakage of current or even inductive influence. In operating X-ray machines never attempt to alter connections or make adjustments other than at coil platinum screw or Crookes tube spark gap without first shutting off current. Remember that a very unpleasant shock can be easily obtained from touching the apparatus with only one hand. It is often advisable to remove one's watch, particularly when using Ruhmkorff coils of large size.
The tube may be worked until it shows a slight redness in the centre of the platinum, but care must then be taken not to increase current, or the platinum will melt. Never allow the tube to come in contact with any object other than its stand and connections while working, and be sure the wires from secondary do not come near tube until they reach places of attachment, or they may spark through glass and ruin the tube.
In making radiographs on sensitized plates the unused plates should be kept at a considerable distance from the coil while working. Better still if they are in another room. Plates for X-ray work are made by most photographic supply dealers; in fact, almost any good brand of sensitized plates or even films will answer. When making a radiograph, the plate can either be left in the holder or well wrapped in black paper, but current should never be turned on coil before the plate and subject are in position. In photographing the chest, neck, etc., the plate can be strapped on to the part; but the subject must remain absolutely still. The time of exposure varies considerably with the size of coil, thickness of object, etc. Radiographs of the hand have been taken by simply laying the hand on top of the plateholder and operating tube for 100 seconds. But, as a rule, longer exposures are necessary. Most radiographs will generally require that the plate be "intensified" and a developer used that gives great detail, such as metol quinol, etc. At any rate, great care should be exercised in developing the plate, as many a good radiograph has been spoiled by undue haste.
CHAPTER XIV.
WIRELESS TELEGRAPHY.
In Chapter XII. we showed how Dr. Hertz caused electric waves to pass through space and become visible by sparks across an air gap in a wire ring situated at a distance from the source of energy. The apparatus used, and termed an electric resonator, is in principle similar to that of the wireless telegraph. The minute sparks instead of idly passing across the air gap are made to traverse a "coherer" (to be afterwards more fully described). This "coherer" substantially consists of a resistance, preferably metal filings placed in series, with a battery and relay. Normally, the resistance is so adjusted that the battery current is not strong enough to operate the relay. A wire is led from one side of this coherer up into the air to intercept the Hertzian waves, the other side of the coherer is put to earth, or "grounded." When a wave strikes the air wire it sends a current through the coherer to ground (as before it sent a spark across the air gap), and this wave acts on the filings in its passage through them; in effect, to lower their resistance, so that the current is increased through the relay circuit and the relay armature is attracted to its magnet. The relay makes contact in the usual manner at the platinum points, and in its turn causes the local circuit, sounder, bell, or pen register to record the wave (or signal). After each wave the filings are in such state that to restore them to their former high resistance it is necessary to give the coherer a smart tap. This is generally accomplished automatically by means of an arm extending from the sounder lever, which strikes against the coherer each time the sounder armature is moved.
Fig. 74.
Figures 74 and 75 are diagrams of a simple circuit, Fig. 74 being the transmitting apparatus and Fig. 75 the receiving apparatus.
In Fig. 74 P P and S S are the primary and secondary of a Ruhmkorff coil, D two brass balls on the discharger, B the battery, K a key, in place of the usual contact breaker, which is either absent or screwed down; V a wire leading from one arm of the discharger up into the air, of a height varying with the results desired; G a ground plate in connection with the other discharger arm.
The coil condenser is left out of the diagram for sake of clearness; but, of course, is necessary to the operation of the apparatus.
In Fig. 75, C is the coherer, also called the Branly tube, or radio conductor; S a telegraph sounder, or electric bell; R a relay; R B and L B the relay battery and local battery, respectively; G a ground connection; M a resistance, or choke coil, and V a vertical wire, as in the transmitter; in fact, in the station set the same vertical wire answers for both transmitter and receiver.
Fig. 75.
The coil to be used may be from two inches of spark upwards, dependent upon the distance the signals have to travel. The relay battery may be two cells of dry battery, the local battery as much as is desired to operate the bell, sounder, or pen register receiving the signals. Presuming the apparatus set up and adjusted, and designating the transmitter as Station A and the receiver as Station B, the operation will be as follows: A pressure and release of key K sends an impulse of current through the primary P, inducing a current in S, which manifests itself by a spark between the discharger balls at D. An electric wave is released, which, starting from V, Station A, meets in its passage V of Station B. Travelling along this wire to the ground, it finds two paths—through C or R. As the choke coil deters it from passing through the relay, it finds passage through C and so to ground.