. Radiography and radio-therapeutics . of attention in the past. For X-ray purposes theordinarv stock type of rotary has been fomid wanting in various points,and a rotary that is of the true and correct type necessary for this workis now used. Alternating Current.—On the alternating current, the problemhas been to secure a good self-starting synchronous motor to drive therectifying disc mechanism. A small, single synchronous motor is a decidedadvance in this respect. It is silent running, brushless, and commutator-less. It starts instantly, and requires but a single switch to put it intocircui
. Radiography and radio-therapeutics . of attention in the past. For X-ray purposes theordinarv stock type of rotary has been fomid wanting in various points,and a rotary that is of the true and correct type necessary for this workis now used. Alternating Current.—On the alternating current, the problemhas been to secure a good self-starting synchronous motor to drive therectifying disc mechanism. A small, single synchronous motor is a decidedadvance in this respect. It is silent running, brushless, and commutator-less. It starts instantly, and requires but a single switch to put it intocircuit. To hold the motor rigidly in place and in constant alignment a goodbase and foundation are necessary. To ensure this an iron platform is 40 EADIOGRAPHY used, mounted on iron supports and thoroughly braced ; no part of themachine except the cabmet itseK is of wood — a vital point to bearin mind. Switchboard.—The control switchboard is made of hishly pohshedslate. Mounted upon it are all control s^^^tches, each one heavy enough. Fig. 28.—Direct current internipterless trausfoniier, showing reiir ot uiachiue iuid switchboardwith large capacity rheostat, serial tinier with mercury dip, X ray meter and lead-lined pro-tection screen attached. An exanunation of this piece of apparatus shows how complicated the mechanical constructionof an X-ray machine has become, and clearly demonstrates the need of a considerable degree oftechnical knowledge on the i)art of the radiologist. to carry the highest loads, fuse-connecting blocks, and high-tension volt-meter. In addition, a large-voltage selector switch is conveniently moimted,so that a change in voltage may be effected immediately. The switchboardis readily detached from the other apparatus and may be placed at any HIGH-TENSION TRANSFORMER 41 desired position. On the alternating current switchboard a polarityindicator is mounted. This adjunct indicates the polarity of the dischargecurrent and enables the operator to predetermi
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