10 EXPERIMENTAL PHYSIOLOGY. 



greater voltage is necessary than if the skin has been moistened, 

 because moist skin is a better conductor. Dry cells are made to 

 give in the neighbourhood of one or two volts. Greater voltage 

 in a circuit can be obtained by connecting two or more cells in 

 series, i.e., zinc to carbon. 



In most of your work the current generated by dynamos in 

 the power plant will be used. At each working place two wires 

 from the main line are connected with a metal ribbon of high 

 resistance. Terminals are arranged so that different proportions 

 of this ribbon can be used in the circuit, thus varying the amount 

 of voltage. The amount of voltage obtained can be estimated by 

 adding the total voltages between the two terminals employed. 

 The following diagram gives the voltages between terminals. 

 T represents the terminals as arranged on the table. 

 T8T8T4T4T4T2T2T 

 abcdef gh 



To illustrate the method of finding voltage, connecting a to f 

 gives 28 volts; f to h, 4 volts; d to e, 4 volts. 



Besides the direct current one can use the induced current for 

 stimulation. It is well known that if one wire is placed near 

 another wire which carries a current, a brief current is induced in 

 the first wire at the time of breaking the circuit in the second, 

 likewise a current is induced at the closing or making of the circuit. 

 The wire carrying the direct current is called the primary circuit, 

 while that in which the current is induced is called the secondary 

 circuit. Greater effects can be produced by having a coil of well 

 insulated wire surrounding a soft iron core as the primary circuit 

 element and another coil of insulated wire as the secondary element. 

 These two coils are arranged on a stand for convenience, the whole 

 apparatus being termed an inductorium (Fig. 1). The chief 

 advantages of the induced over the direct current for purposes of 

 stimulation are; first, the voltage is very much greater so that 

 high resistances can be overcome and, secondly, it can readily be 

 altered in strength. 



Note that the inductorium is so arranged that by connecting 

 binding post 1 and 2 a single momentary current is induced in the 

 secondary coil when the circuit is either made or broken in the 

 primary coil; and that by connecting binding posts 1 and 3 brief 



