. Elementary and dental radiography / by Howard Riley Raper . coil to thetarget end of the tube, we must be able to determine which is the positiveterminal of the coil. This may be done on an induction coil, as follows:Cut out the resistance of the rheostat, adjust the sliding rods to aboutone-half the distance of the maximum spark gap, and throw on theswitch. The spark will jump the gap so quickly that it is impossible tolearn by simple observation in which direction it is traveling. By watch- 4i 42 ELEMENTARY RADIOGRAPHY ing the large disc terminal, however, this can be determined. If onthro
. Elementary and dental radiography / by Howard Riley Raper . coil to thetarget end of the tube, we must be able to determine which is the positiveterminal of the coil. This may be done on an induction coil, as follows:Cut out the resistance of the rheostat, adjust the sliding rods to aboutone-half the distance of the maximum spark gap, and throw on theswitch. The spark will jump the gap so quickly that it is impossible tolearn by simple observation in which direction it is traveling. By watch- 4i 42 ELEMENTARY RADIOGRAPHY ing the large disc terminal, however, this can be determined. If onthrowing the switch on and off the spark is noticed to cling to the edgeof the disc always, then the current is passing from the disc. If, how-ever, the spark occurs from the surface of the disc just as the current isturned on (it may then seek the edges), the current is traveling from thesmall bulb to the disc. ( Fig. 40.) With the tube properly connected to the coil as per Fig. 41, thecurrent is shunted (Fig. 42) through the tube, instead of jumping the «fc. Fig. 39. A, anode. B, cathode. C, point at which the atmosphere was pumped from the tube. spark gap, passing from anode to cathode. Whether the current will choose the path through the tube or jump the spark gap depends on which offers the less resistance. A current of electricity always travels the path of least resistance. _, ... Tubes are designated according to the degree of Classification of _, s J» ° X-Rav Cubes their vacuum, thus we have the high or hard tube. According to Degree in which the vacuum is well-nigh complete; the me-et Uacuum dium tube in which the vacuum is less complete, andthe soft or low tube, in which the vacuum is least complete. High tubesoffer the greatest resistance to the passage of the electric current throughthem, then comes the medium, while the low vacuum tube offers the leastresistance. For dental picture work a medium tube yields excellent operator may determine whether a tub
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