. Elementary and dental radiography . t is, we motors which can be run only by a direct current, and motorswhich can be excited only by an alternating current. Instead of a rotary converter, some machines are equipped with amechanical vibrator (Fig. 22), which interrupts the current as it entersthe coil. This is not so efficacious as the converter. Machines with themechanical interrupter are advertised to operate on either or The length of the spark gaps of portable machines varies from fourto ten inches. They are seldom able to give a fat, fuzzy spark longerth
. Elementary and dental radiography . t is, we motors which can be run only by a direct current, and motorswhich can be excited only by an alternating current. Instead of a rotary converter, some machines are equipped with amechanical vibrator (Fig. 22), which interrupts the current as it entersthe coil. This is not so efficacious as the converter. Machines with themechanical interrupter are advertised to operate on either or The length of the spark gaps of portable machines varies from fourto ten inches. They are seldom able to give a fat, fuzzy spark longerthan half the length of the spark gap. The full-length spark is almostalwavs thin and blue. 38 ELEMENTARY RADIOGRAPHY As a general proposition the alternating is the Danger of most dangerous of electric currents. Mysterious and Currents. surprising as it is, however, high-frequency currents, such as high-frequency coils produce, are much less dangerous than the current of an induction coil. If one should touch a Line,J10 Direct Current. Portable Coil. converter Fig. 37. terminal of a high-frequencj coil a spark would jump to the hand justas it came within sparking distance of the terminal. This spark mightmake a blister, hut if the experimenter had been standing on a non-con-ductor, no further discomfort would he felt. Indeed, one may take asmall steel bar ami. standing on a non-conductor, touch a terminal of oil and -o take into the body enough current to cause great injury,if it were of the nature of the induction coil, without receiving any sen-sation at all. If he should stand on a conductor, however, the shockwould he painful. If he should grasp both terminals, so that the entirecurrent passed through the- body, the result would be severe pain andprobably injury. Tin- larger the coil the more dangerous the current. The primary current of the portable high-frequency coils is not asdangerous as the primary current of the stationary induction coils, he- X-RAY MACHINES 39 cause
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