Wheatstone Bridge Network Research Articles

Calorimeter with automatic control

The temperature differences between the specimen and its surrounding reference shield in a Smith's calorimeter, modified to operate in a vacuum, is held constant by automatically adjusting the power input to the furnace. Fine adjustment is achieved by a variable reactance in the power supply. The reactance control current is obtained from a single valve d.c. amplifier, the control signal of which is the output voltage of a Wheatstone bridge network containing two photocells. Departures from the desired temperature difference cause a light spot reflected by a galvanometer to move from one to the other photocell, and so adjust the furnace power input and restore the original conditions. Specific heat-temperature curves smooth to within ±0.5% and reproducible within 3% have been obtained between 50-250° C.

A Fast-Response Resistance Thermometer System for Simultaneous Recording of Air Temperatures at a Number of Separated Points1

A resistance-thermometer system is described which permits simultaneous and continuous recording of air temperatures at a number of separated points. Unbalance current in a Wheatstone bridge network is used as the measure of temperature, and is recorded by a multichannel light-beam oscillograph. The lag coefficient of the system is approximately 0.1 second.

A simple temperature-controlled laboratory furnace

A simple and robust laboratory furnace using an on/off method of control is described. The temperature-sensitive element (or thermometer) is a platinum resistor which forms one part of the heater winding of the furnace, the other part, which is in series, being a nickel-chromium resistor. The simplicity of the furnace control derives from this combination of heater winding and thermometer, since such an arrangement calls for little amplification in the control circuit. The resistors of the heater circuit act as two arms of a Wheatstone bridge network which is balanced at a temperature determined only by the setting of the inductive ratio arms. The control circuit, which maintains the bridge in balance, consists of a thyratron operated switch, the only amplification of the out-of-balance signal from the bridge being provided by an audio-frequency transformer with a tuned secondary. For more than six months the apparatus has given satisfactory service at temperatures up to 800° C. For example, in a run at 750° C the temperature was maintained constant to ± 0.1° C for a day.

The Thermal Volume Meter

The thermal volume meter was originally brought out by Professor Thomas, and is frequently called a Thomas meter. The principle is that the temperature of the gas is raised by means of an electrical heater, the change in temperature being accurately measured either by means of resistance thermometers connected in a wheatstone bridge network, or by means of a thermopile. It is believed that the thermocouple method is preferable to resistance thermometers, and most of the paper deals with that construction. The usual construction has walls of wood or micarta, which serve as an insulating support for the heater coils and thermocouples. The general proportions of the volume meter as to section and length are dealt with, the former being determined mainly by the mean velocity of the air, and the latter by the distance needed to give the gas an opportunity to mix after passing the heater. A sufficient number of thermocouples is required to give a good reading of the potentiometer with a comparatively low temperature rise of the gas and also to obtain a good average of the gas temperature. The details of construction are described. The equations that are suitable for calculating the volumes of the gas are then given. Then follows considerable discussion on the possible sources of error in the meter, the principal ones being The non-uniform heating of the gas. Non-uniform distribution of velocities. Heat escapement. Methods are suggested for determining the magnitude of these various conditions experimentally or by calculation.