The Peltier Effect and its Use in the Measurement of Suction Pressure

1. A method is described by means of which the suction pressureof living material may be measured by observations on its equilibriumvapour pressure.
2. It employs a very fine thermocouple whosetip is cooled by meansof the Peltier effect, below the dew-point.It collects a minuteamount of dew, and can thus be used asan exceedingly fine ‘wet-bulb’thermometer. As suchit is convenient to use, quick in action,and causes very littledisturbance to the system under observation.
3. The thermocoupleis calibrated by means of salt solutions ofknown strength.The scale is fairly linear.
4. The method possesses the advantagesof giving the suction pressureby direct observation ratherthan by interpolation; of speedin the actual measurement; ofnot subjecting the tissue to immersion;and of permitting organslike leaves, otherwise difficult tohandle, to be easily dealtwith. It also opens possibilitiesoutside the range of the conventionalmethods.