SECTIONAL, TRANSACTIONS.— A. 355 



where d = the sum of the distances of each wire from the round and oval 

 windows, and b = the breadth of the wire. This formula is an adaptation of 

 the formula 



n= V 

 •21 m 



to strings immersed and oriented as in the cochlea. The whole is completely 

 filled with water. It is set into localised resonant action in the calculated posi- 

 tions by applying tuning forks to the ' stapes.' Its compass is four octaves. 



(cl) By The Cambridge and Paul Scientific Instrument Com- 

 pany, i. Instrument for Measuring the Percentage of Carbon- 

 dioxide in Alveolar Air. 

 To physiologists working on respiration and the respiratory functions of the 

 blood it is important to be able to measure the carbon-dioxide in alveolar air. 

 In diabetes, for example, a lowering of the carbon-dioxide is an indication of 

 danger, and the percentage figure is therefore a valuable guide to prognosis and 

 treatment. The instrument shown, suggested by Professor A. V. Hill, provides 

 a convenient and accurate way of measuring this percentage. The patient 

 breathes out in the ordinary way, and then, by an effort, expels through a 

 katharometer of the Shakespear type the residue of the gas in the lungs which 

 has been in actual contact with the blood system. The percentage of CO2 in 

 this residue is then read on the indicator scale, which is calibrated _ 10 per 

 cent. CO2. The whole outfit is contained in a portable case. 



ii. Salomonson String Galvanometer. 

 This is a simple form of Einthoven galvanometer. It has two copper 

 fibres, 0.01 in. diameter. The magnification is about 40 at a working distance 

 of 80 cms. The galvanometer can be used in conjunction with a larger Einthoven 

 galvanometer (fitted with a silvered glass fibre), the optical work in the small 

 mstrument forming the eye-piece of the large one. An electro-cardiogram, a 

 phono-cardiogram, and a pulse-tracing can thus be taken simultaneously on 

 the same plate. 



iii. Hydrogen Ion Apparatus . 



The E.M.F. is measured by a potentiometer, in which the slide wire, con- 

 tacts, and resistances are totally enclosed, thus being protected from corrosion 

 and other deteriorating effects. The instrument reads directly to 0.2 milli- 

 volts. It is connected to a moving coil galvanometer, the movement of the 

 coil being observed by a lamp and scale. The potentiometer circuit is stan- 

 dardised by a Weston normal cell. The electrodes are of the Clark pattern, 

 and comprise two hydrogen electrodes, a connecting \ essel, and calomel electrode, 

 suitably mounted with rocker and motor. 



For blood work a special set of electrodes is supplied. 



For electrometric titration work a direct-reading potentiometer is supplied, 

 the instrument consisting of a moving coil galvanometer of the pivoted type, 

 adjustable rheostat, battery, and electrode reversing switch. The E.M.F. of 

 the electrodes is read directly on the scale of the instrument, which is calibrated 

 in millivolts, the standard ranges being 0-600 and 0-1,200 millivolts. 



iv. MacGregor-Morris Anem,ometer. 

 This is an electrical anemometer designed for measuring the velocity of slow- 

 moving air currents. The instrument consists of a form of Wheatstone bridge, 

 in which two of the arms are made of fine nickel wire. These wires are heated 

 by the passage of an electric current of constant strength, the wire attaining 

 a steady temperature in a few seconds. The heat is then taken away from one 

 of the wires by the motion of the air passing over it, the second wire being 

 protected from the moving air. The heat carried away from the exposed wire 

 by the air current is proportional to the square root of the velocity of the 

 wind. The amount the bridge is out of balance is shown by a direct-deflection 

 galvanometer, the scale of which is calibrated to read directly in air velocities. 

 The method is a sensitive and accurate one, over a range of velocity from 

 200 to 2,000 cm. per second. 



