October 16, 1914] 



SCIENCE 



569 



resistant as copper and carrying 11 amperes 

 will give a heat rise of about 160° F. The 

 length is less than 200 feet. In selecting wire 

 for conditions other than those given above, 

 the different wire capacity tables may be con- 

 sulted for various heat rises, lengths, etc., that 

 are easily obtained from wire manufacturers. 

 The choice will lie mainly with iron, 18 per 

 cent, and 28 per cent. German silver, " climax " 

 and nickel-chromium wires or their equiva- 

 lents given under various trade names. Their 

 resistances are, respectively, seven, twenty, 

 thirty, fifty and sixty times that of copper. 



Fourth, Unit System Installed. — The " indi- 

 vidual unit " system, as previously mentioned, 

 carrying 2 amperes, is applicable for a 

 limited number of certain instruments, partic- 

 ularly those of higher resistance. Several 

 such systems are necessary for class work. 

 Jackson's^ " single unit " system consists 

 essentially of one large frame over which is 

 strung the resistance line, and has a capacity 

 for a large number of instruments. This has 

 in general all the favorable points of the shunt 

 rheocord system, but the chief drawback is 

 that such a frame is situated at one place 

 from which all tapping wires must lead. In 

 class work this may incur confusion in iden- 

 tifying individual tappings, and more espe- 

 cially necessitates the running of an excessive 

 amount of wire from the frame to each table. 

 Further, it is desirable that each machine, 

 particularly inductoria, has its own separate 

 connection to the resistance board in order that 

 its operating current may be varied at will and 

 may not be affected by the working of any 

 other instrument, as is the case when one or 

 more are placed in parallel with it. The 

 " multiple unit " system eliminates this ob- 

 jection to the " single unit " by dividing the 

 latter into several sectional units connected 

 in series and placing one section near each 

 table. Confusion is avoided, extensive wiring 

 unnecessary, and quick variations of currents 

 to individual instruments readily made. 



Fifth, Miscellaneous Details. — In general, 

 these are for convenience and safety and con- 



2 Jackson, Journal A. M. A:, 1912, Vol. LVIII , 

 p. 1011. 



cerned with electrical rules and regulations. 

 The main leads and the wires connecting the 

 sectional units should be insulated copper 

 large enough to carry the desired current 

 (B. & S. No. 16—6 amperes, TSo. 14—12 am- 

 peres and No. 12 — 15 amperes). All connec- 

 tions are thoroughly fastened or spliced and 

 soldered if necessary. 



Sectional or individual units may be con- 

 structed to suit individual preferences, the only 

 requirement being proper insulation of the 

 bare wire. Stringing the resistance line over 

 wooden frames, even asbestos lined, is not 

 always advisable because of possible dangers 

 from accidental overheating. Slabs of slate 

 or stone are more preferable since they permit 

 arnple insulation and protection. The resist- 

 ant units in the author's "multiple unit" 

 system are slate slabs 14 in. x 12 in. x 1 in. in 

 size with a i-in. beveled edge, a ^-in. hole 

 hear each corner for fastening unit to the 

 wall being separated from it by 2-in. porcelain 

 Spools. One inch in along each long side a row 

 of holes is drilled to fit 3/16 in. stove bolts, 

 the holes being -|- in. apart and so located that 

 the wire when strung shall run in a zigzag 

 manner. Through the holes bolts are inserted 

 from the rear surface; a washer is placed on 

 each next to the slate on the front surface; 

 and the wire is strung tightly from bolt to bolt, 

 each of which is finally tightened by a single 

 washer and nut. The bolt ends should project 

 out free J in. so that spring clips of the tapping 

 wires may be easily attached where direct wire 

 tapping is less convenient or not desired. Wire 

 strands between bolts are 10 in. long and each 

 strand produces approximately a .5-v61t drop in 

 the current. Thus a 2-volt drop is obtained 

 across four strands. If tappings are to be 

 made from the bolt ends only, the resistance 

 wire may be coiled spirally, thus shortening 

 the span of the strands and materially dimin- 

 ishing the size of the units. 



Tapping wires are twisted flexible lamp cord 

 of ten or other convenient length with ends 

 numbered and all lightly soldered to prevent 

 the strands from breaking and with spring 

 clips, fastened to one pair of ends, for attach- 

 ing to the bolt ends or the resistance wire. 



