STIMULATING ELECTRODE 



213 



primary current there is a corresponding capacitance which makes the po- 

 tential across the terminals of the spark gap a maximum. The primary 

 current I p rises exponentially on the closing 

 of the interrupter, as shown in Fig. VI-1. 

 When the circuit is suddenly opened at C 

 the current rapidly drops to D. Without 

 the condenser this slope would be less steep. 

 Since the current builds up more slowly 

 than it diminishes, the associated magnetic 

 field builds up more slowly than it col- 

 lapses. The induced electromotive force, 

 and therefore the induced current in the 

 secondary, are smaller when the primary cir- 

 cuit is closed than when it is open. This 



/ Time 



— 0.01 sec ~ 



Fig. VI-1. An ideal rise 

 and fall of the current in the 

 primary and accompanying 

 changes in current in the sec- 

 ondary of an induction coil 

 condition is illustrated by the broken-line with proper condenser con- 

 curve I s . The larger induced electromotive nected across the mechanical 

 force at the break therefore lasts a shorter mterru P ter - 

 time than the smaller induced electromotive force at the make. 



The voltage across the terminals of the secondary, to which a stimu- 

 lating electrode may be attached, is alternately positive and negative, 

 but one cycle does not consist of two equal alternations of opposite sign. 

 Therefore the discharge across the terminals of the stimulating electrode 

 is, as seen in Fig. VI-1, predominantly in one direction. 



pt 



2 cm 



Stimulating Electrode 



The electric stimulus can be applied to the irritable tissue with the aid 

 of the two-prongecl metal electrode shown in Fig. VI-2. The binding 



posts B are attached to the high-po- 

 tential terminals of an induction coil 

 or variable-frequency stimulator. 



The handle S is an insulating cover 



Fig. VI-2. A form of stimulating of hard mbber tQ faciHtate han . 



electrode m which the electrodes are „. t->i , • n r>. i 



pronged into a holder including a cord dhn %- Platinum needles, Pt, spaced 



about 2.5 mm, are placed in direct 

 contact with the tissue. 



If the response of an excised nerve 

 is to be investigated, the platinum 

 needle-electrodes are placed across 

 and near one end of the preparation. 

 When an adequate electrical discharge passes between the terminals of 

 the electrodes, an excitatory impulse travels away from the point of 

 stimulation. 



and plug. They can be sterilized. 

 Overall length 4 in., tip length 1 in., 

 tip spacing 0.1 in. Also available with 

 longer electrodes. (By courtesy of 

 Allen B. DuMont Laboratories, Passaic, 

 New Jersey.) 



