Proceedings of the Physiological Society

Abstract

Many students are helped by seeing Lillie's model. An alternative electrolyte has been found which is less noxious than strong nitric acid with its fumes, gives a cleaner reaction, and permits greater control of the conditions. The original model consisted of an iron wire rendered 'passive' in strong nitric acid (Lillie, 1918(Lillie, , 1919(Lillie, , 1920(Lillie, , 1922. The wire is 'stimulated' with a piece of zinc, or by other chemical, mechanical or electrical methods. Refractory period, threshold phenomena, summation, electrotonus, etc., can all be imitated.The action of the acid is twofold: (1) it causes oxidation, depositing a film on the wire, (2) it acts as an acid, attacking the wire at the active portion. A mixture of hydrogen peroxide solution and nitric acid in which the proportion of acid is low (c. 3 %) produces the same result, is cleaner, and allows the two effects to be controlled by varying the proportions in the mixture.To obtain a slow impulse which is easy to see, thick wire should be used (gauge 10). It should be relatively pure, commercial galvanized iron wire being suitable if the galvanizing is first removed with dilute hydrochloric acid; the wire is then washed in water and wiped on a clean cloth. Place the iron wire in a flat dish, and cover with a mixture of hydrogen peroxide (20 vol.) and concentrated nitric acid in the proportion 30:1. Add more acid drop by drop, testing the wire with a piece of zinc, until satisfactory propagation is achieved. If too much acid is added, causing the wire to effervesce continuously, more peroxide can be added. a