570 



SCIENCE 



[N. S. Vol. XL. No. 1033 



Into the tapping wires between the spring clips 

 and instrument connections J or 1 ampere 

 fuses, which "blow out" with IJ to 2J am- 

 peres of current, may be inserted. Provision 

 is made for connecting in series one or, in 

 some tapping sets, two instruments. 



The system may be briefly described as fol- 

 lows : The " multiple unit " system, used in 

 the pharmacology laboratory of the North- 

 western University Medical School consists of 

 8 sectional units, connected in series, strung 

 with 10 ohms of No. 15 B. & S. German silver 

 18 per cent, nickel alloy wire about 200 feet 

 long. The 110-volt, 11-ampere current enters 

 at the positive main, passes through a cartridge 

 fuse and switch on an enclosed switchboard, to 

 resistance unit No. 1, to unit No. 2, so on 

 consecutively to unit No. 8, and back through 

 the switch and a fuse to the negative main. 

 A pilot light is connected in parallel across 

 some unit to indicate when current passes 

 through the line. From varying points on any 

 unit, double-fused flexible lamp cord may be 

 led off to an inductorium. Similarly, a signal 

 magnet, or an inductorium with a signal 

 magnet in series, may be connected. Each 

 strand is 10 in. long and has a .5-volt poten- 

 tial. Single instruments operate across a 3 or 4 

 strand shunt (1.5 to 2 volts), two instruments 

 in series operate across a 4 to 6 strand shunt 

 (2 to 3 volts). All instrument circuits take 

 :5 to 1.5 amperes, according to their resistance, 

 while during the passage of the current the 

 voltage drops from .05 to .3 volt, due to the 

 decreased resistance across the shunt. It is 

 wise to test each instrument, because of pos- 

 sible differences in its resistance, with the 

 volt-meter and the ammeter before using it 

 in regular work. The "multiple unit " system 

 is likewise admirably adapted not only for 

 tissue stimulation with the direct current as 

 previously mentioned, but also for physiologic 

 chemical work as the determination of copper 

 in sugar analysis, etc. The cost of such an 

 outfit will range between 5 and 15 dollars, in- 

 cluding units, switch box, wire and tapping 

 cords. Since the operating expense is but a 

 few cents per hour and the " system " is a 

 permanent fixture, the actual expense is much 



less than that of dry batteries, which must be 

 frequently renewed. 



A few possible dangers are to be remem- 

 bered. If the negative main be connected to 

 the ground, as occurs with some power plants, 

 " grounding " of the positive main from any 

 point along the resistance line may take place 

 through a tapping wire, either directly by con- 

 tact with water pipes, radiators, etc., or in- 

 directly through instruments not insulated 

 from stands which themselves are grounded. 

 In either case the grounding wire and any 

 instrument in series with it takes part of the 

 line current which usually bums out the 

 small fuses in the tapping wire but, if not, 

 may be so large as to injure the instrument. 

 Signal magnets, if not insulated, may " short 

 circuit " by permitting the current to flow from 

 one instrument to another, either through a 

 common stand rod, or through metal writing 

 levers touching a kymograph drum not covered 

 with tracing paper. This will prevent the pas- 

 sage of sufficient current through the instru- 

 ments which then do not work properly. With 

 a 2-ampere system for 3 to 4 instrument 

 capacity, only the last 8 or 10 ohms of the 

 wire nearest the negative main should be used. 

 This, as weU as the fusing of the individual 

 tapping wires, minimizes the danger. Like- 

 wise, it is preferable if possible to have instru- 

 ments operated on the negative side of a larger 

 ampere line in order to reduce the seriousness 

 of grounding. Students should be given the 

 following instructions to prevent these occur- 

 rences. 



First, always make sure that the line has no 

 possible " ground " before the main current 

 is switched on. 



Second, tap last from a resistance unit when 

 setting up an apparatus and disconnect iirst 

 from the unit when changing instruments or 

 through using apparatus. 



Third, insulate signal magnets and other 

 electrical apparatus from metal stands by 

 heavy rubber tubing and keep tracing paper on 

 drums which are in contact with metal writing 

 levers. q_ l_ y_ jjess 



Northwestern University, 

 Medical School 



