. Elementary and dental radiography . to force enough milliamperagethrc^ugh it. b^rom the foregoing it will l)e seen that any coil smaller than onewith an eight-inch spark gap could not well excite a high tube, and thatat least a ten-inch ccmI is necessary to light a very high tube. It , that any coil with a spark gap wider than eight or ten inches is need-lessly large. The coils with the long spark gaj)s are. however, seldomable to throw a fat. fuzzy s])ark farther than eight or ten inches. Thethrowing of a thin, blue spark a greater distance is simply incidental anil 44 ELEMENTARY R
. Elementary and dental radiography . to force enough milliamperagethrc^ugh it. b^rom the foregoing it will l)e seen that any coil smaller than onewith an eight-inch spark gap could not well excite a high tube, and thatat least a ten-inch ccmI is necessary to light a very high tube. It , that any coil with a spark gap wider than eight or ten inches is need-lessly large. The coils with the long spark gaj)s are. however, seldomable to throw a fat. fuzzy s])ark farther than eight or ten inches. Thethrowing of a thin, blue spark a greater distance is simply incidental anil 44 ELEMENTARY RADIOGRAPHY without practical usefulness. Thus an eight or ten-inch coil may be aspowerful as one with an eighteen or thirty-inch spark gap; that is,capable of forcing as high a milliamperage through a high tube. If,however, a coil can force any kind of a spark at all through from eighteento thirty inches of atmosphere, we may be sure it will send a high milliam-perage through a good radiographic tube, or, what is the tubes equivalent. Fig. 41. The X-ray tube connected with the induction coil. in resistance, six or eight inches of atmosphere. It is so well understoodto-day that the coil with the very wide spark gap is not necessarily morepowerful, that manufacturers are making practically all of their coilswith from an eight to a twelve-inch spark gap, then rating them ac-cording to the milliamperage they can force through this resistance. To light a tube well a coil should be capable of giving a fat, fuzzyspark, the distance of the parallel spark which the tube backs up. The tube thus far described is the simplest form of X-ray in simplicity is the bi-anodal tube. (Fig. 43.) When the two anodes are connected, as in Fig. Bi-anodal 43. the positive terminal may be attached to either tubes. anode or assistant anode, preferably the anode. The advantage of the assistant anode is a matter on which authorities have widely different opinions. One manufacturer, a man X-RAV TUB
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