Page:Popular Science Monthly Volume 89.djvu/784

 770

��Popular Science Monthhj

���Fig. 7. Apart of the slide-carrier holder

��bottom of the pot and into the brass bracket. The drawing shows how the disk of clear glass is clamped to the front of the flower pot. The method of constructing the lamp-mounting, wiiich slides over the brass tubes of the instrument frame, is clearly indicated in Fig. 14. The addition of the rear sup- port, detailed in Fig. 15, completes the stereopticon. For the sake of completeness, the details of a small rheostat are given in Fig. 16. While a lamp-bank, or any other form of re- sistance that will limit the flow of cur- rent to about 5 amperes, may be used, the wire resis- tance illustrated will be found inexpensive and compact. The base is a piece of slate about 8 in. square in which are mounted nine brass pillars. On cither side and between these pil- lars are stretched

9 coils of No. 22 gage resistance wire, each coil containing 5 fl- of the wire. If wound in coils of 48 tu.-s each on a rod

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� ��Fig. 8. Another part of slide-carrier holder

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��*/2 brass w/rs.

��approximately correct. As the drawing shows, the coils arc all placed in series.

In Fig. 17 is given the diagram of connections. The wires leading from the nearest lamji- socket connect with a switch and fuse. One wire goes directly to the upper carbon of the lamp, and the other wire goes to one ter- minal of the rheostat. l'"roin Fig. 10. Brass wire ,1,,. ^iiond ter-

rings used to j, j ^ ,^ ^.

strength cnthc numerous joints rheostat, a WMC

���connects with the lower carbon of the lamp. If the current is direct, the upper or horizontal carbon should be the positive. This can be insured by a simple test; turn on the current at the switch and bring the carbons together for an instant, separating them im- mediately for ' is in. If the current is direct, the arc will cpiickly settle down to an absolutely noiseless burning; if

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��Fig. 9. Dimensions of the uppyer and lower frame parts of the slide-carrier holder

alternating, the arc will emit a low hum. If direct current is used, turn the current ofif after the arc has burned a few moments and immediately observe the carbons; the positive will be the hotter one and if this should, by chance, be the lower, the wires at the lamp terminals should be transposed so as to establish the positive pole at the upper carbon. With a direct current arc, practically all (jf the light comes from the positive carbon, while with an alternating current, both carbons give off the same amount of light.

When the light has been tested, the stereopticon may be set up for use. Place the instrument upon a firm table and at right angles to the screen, which

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��in diameter, the a.nount will be Z-

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��� ��4 reg. brass

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�� ��4 rcg. brass

��4 rcg. brass Fig. 11. Dimensions of the collars which arc forced over the tubes and sol- dered in place gripping the lens-board

may be a slieet or a bare white wall. Start the light at the arc and darken the room. I'lace a slide in the carrier and bring the lens into focus by sliding the

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