October 16, 1914] 



SCIENCE 



567 



perimental work, lias been satisfactorily 

 solved in but few laboratories. Very little on 

 the subject is found in the literature and the 

 need of a practical method which is compre- 

 hensive and can be intelligently adopted, is 

 becoming apparent. With this in mind the 

 writer presents a brief discussion of the 

 sources of electricity suitable to laboratory 

 use, with special reference to what he terms 

 the " multiple unit " system. 



Dry batteries are extensively used chiefly 

 because of their compactness, ease in handling 

 and apparent cheapness. But they are not 

 dependable, since they polarize easily, the cur- 

 rent is not constant and the supply is limited. 

 Because of this much time is often lost in 

 getting apparatus to work properly. In addi- 

 tion the cost per year is usually a considerable 

 item. Tet in spite of these inconveniences 

 they still remain the common source of elec- 

 trical supply. Wet batteries have the same 

 disadvantages as dry cells. They are also 

 clumsy and hence little used. Storage cells 

 are fairly reliable but their bulkiness and 

 expense make them undesirable for student 

 work. 



The direct electric lighting current is an 

 excellent source. A suitable resistance wire 

 is attached in series to this as a rheocord from 

 which sufficient current may be tapped off at 

 various points and led to different instruments. 

 The principle involved is well known, although 

 it appears that but few physiologic or phar- 

 macologic laboratories are utilizing it. This 

 shunt rheocord system has the advantage of 

 being absolutely reliable. The ciirrent is of 

 unlimited supply and the voltage or amperage 

 can be either made constant or varied at wiU. 

 This is important in the stimulation of tissues 

 with the direct current, where graded amounts 

 are desired. Such an outfit may be made com- 

 pact, accessible and inexpensive; it requires 

 little care and will last indefinitely. 



The installation of such a system involves 

 several important considerations. 



First, Source. — ^Preferably, a direct 110-volt 

 current should be used. 



Second, Amperage Carried. — This is deter- 

 mined largely by (a) the amount of current 



necessary to make any instrument work prop- 

 erly, (b) the internal resistance of each, and 

 (c) the number of instruments to be used and 

 their effect upon the line amperage when 

 shunted into the line resistance. Most induc- 

 toria of American make operate best with a 

 current of .5 to 1 ampere and 1.5 to 2 volts. 

 The Harvard coil has an internal resistance of 

 about .5 ohm, but this may rise as high as 1 

 ohm with the interrupter in series if the con- 

 tact points of the latter are poor. The 

 Stoelting make No. Y090 has 1.5 ohms, and 

 2 ohms or more with the interrupter. Signal 

 magnets all work well with 1.5 to 2 volts and 

 .5 to 1.5 amperes. Their resistance ranges 

 between .5 ohm and 3 or more ohms (Stoelting 

 No. Y076— .5 ohm; Harvard— 3 ohms). An 

 induction coil in series with a magnet requires 

 a 2 to 3 volt and a .4 to 1 ampere current. An 

 average resistance of all the instruments is 

 about 1.5 ohms. Practically, the above amper- 

 ages may be decreased within certain limits if 

 the voltages are correspondingly increased, 

 and vice versa. Individual needs will deter- 

 mine the number of instruments to be used. 

 In this laboratory accommodations are pro- 

 vided for sections of thirty-five students each, 

 and a maximum of sixty-five instruments is 

 permitted. 



Great increases in the line current must be 

 avoided, and in order to determine the current 

 necessary to keep this rise in the line amperage 

 below any desired maximum, say 15 per cent., 

 it is of advantage to keep in mind the follow- 

 ing formulas: 



The current in amperes (i) equals the 

 potential in volts (e) divided by the resistance 

 in ohms (r). 



i = - 01 e = ir (1) 



r 



The conductance of two wires in parallel 

 equals the sum of the two separate conduct- 

 ances, conductance being the inverse of re- 

 sistance. 



1 1,1 



- = — -f- -77 or 



r' + r" 



(2) 



The amount of current passing through 

 each of two wires in parallel is inversely pro- 

 portional to its resistance. 



