ELECTRIC WIRING SYSTEM 



471 



and a good supply of artificial light should be provided, 

 especially in the form of drop lights, etc., placed near the 

 work benches, the lathe, or other special machines. If 

 only a small room can be provided for the shop (as is 

 often the case) then the arrangement of the machinery, 



jRe/ay 



Fig. 358. A schematic representation of an electric wiring system for using an ordi- 

 nary 110 volt, direct current for regular laboratory purposes, i. e., for induction coils, 

 signal magnets, etc. Beginning at C. and ending at Y, is a resistance frame made of a 

 large number of strips of thin tinned iron ( fjj inch x 28 inches). These strips'are joined 

 together at alternate ends so the current passes in series through the total length of all 

 of the strips. Small coils of special resistance wire may be used for this purpose and is 

 more compact. If "tin" strips are used these should be arranged on frames, each frame 

 holding about SO or 60 strips (the strips are 28 inches long) and about 7 such frames 

 should be placed side by side in an insulated cabinet. All frames are connected together 

 in series and the current leaving at Y in the illustration passes through the switch and 

 thence forms a loop below the pilot light which is connected in parallel (shunt) to the 

 main current wires. The current leaves the frame at Z. Small currents for inductoria, 

 etc., are picked off from the main frame at various points shown at fg, hi, inn, etc. The 

 drop (lamp cord) wires (;', /) pass to the tables for the inductoria, signal magnets, etc. 

 The strength of current going to each table can be controlled at will by varying (by 

 means of a sliding contact) the distance between fg, or hi, etc. At inn. a current is 

 taken off for the clock circuit. In series with one of these wires is placed the master 

 time clock. It is advisable to place a (storage) battery and small (telegraphic) relay in 

 this circuit to protect the delicate contacts of the clock which may be burned out by too 

 strong a current. From the main time circuit wires, O P, the signal magnets, manometer 

 base line signals, etc., receive the time signal currents. Signal magnets, etc., must be 

 insulated from the rest of the apparatus if tlic main feed ivires to the building are 

 grounded at anv place. The wood board of the manometer suffices for this instrument 

 here, but metal signal magnets may be insulated by small pieces of wood fiber tubing (S in 

 the figure) which are held by the double clamps. Also metal writing points of signal 

 magnets should not be brought against drums until the paper has been pasted on the drum. 

 (For further details, see Jour. Amer. Med. Assoc., 1912, 58, 1011; also see Von Hess, 

 Science, 1914, 40, 566; also Yandell, Henderson, ibid., 1915. 41, 910, and McPeek, Reed 

 and Beck, Journal of Laboratory and Clinical Medicine, 1916, ii. 139.) Alternating cur- 

 rent is not suitable for this work unless it can be changed to a direct current by a special 

 transformer (see catalogues of electrical supply houses). 



