GB910000A - Improvements relating to apparatus for determining the melting point of a material - Google Patents

Abstract

910,000. Melting-point determination. PERKIN-ELMER Ltd. July 28, 1960 [Aug. 7, 1959], No. 27113/59. Class 40 (1). The melting point of a sample is determined by means of a heater for the sample and an electric thermometer (resistance or thermocouple) connected in a circuit which is unbalanced while the temperature of the sample is rising but becomes balanced when the temperature remains stationary at the melting point. The heater may be a separate radiator, or may be a resistance wire carrying a horizontal plate to receive the sample and thermometer, but it is preferred to use the thermometer itself as heater by passing a current through it. To avoid errors due to temperature differences within the sample, the heater is preferably disconnected when the circuit becomes balanced, and after a short time delay a record is made of the temperature. In the Figure the hot junction 6 of a thermocouple is in contact with the sample and is heated by current from a transformer 1 when a push-button 4 is pressed. The circuit is maintained by the closed relay contacts 38 (see below). A coil 23 is supplied with A.C. and causes the contact 8 to oscillate between fixed contacts 9 and 10, so that the thermocouple is connected alternately with the heating and measuring circuits. The voltage across the transformer primary 17 (which is chopped by the action of the coil 23) is the algebraic sum of the p.d. between the hot and cold junctions 6 and 16 and the p.d. across the part of the potentiometer 21 to the left of the slider, the latter p.d. coming from a D.C. source 45, stabilized with a tube 50. Any output from the transformer is amplified at 27 and applied to a servomotor 28 to drive the potentiometer slider towards balance, but while the temperature of the sample is rising the slider will lag behind its balance point. At the melting point the temperature will become stationary and balance will be reached. When this happens, the rectifier 35 no longer supplies relays 36 and 37 and these are de-energized. Relay 36 opens contacts 38 to stop the heating, while relay 37 opens contacts 40 to terminate charging of a capacitor 43 from the source 45. After a short time-delay, contacts 41 close, and the charge on the capacitor is applied via stylus 30 to mark heat-sensitive paper 31 which is mounted on a calibrated glass plate 32. The stylus 30 is ganged to the slider of the potentiometer 21, and the latter is wound on a glass rod of the same material as the plate 32 to avoid errors due to ambient temperature changes. After the measurement, the sample can be burnt off the thermocouple 6 by passing a heavy current. Alternatively, the sample is placed in a platinum cup secured to the junction, and covered by a glass slide. The junction 7 can be in vacuo and cooled by ice. The apparatus can be calibrated by using a substance of known melting point, holding the button 4 depressed, and adjusting rheostat 51 until the stylus 30 is in the correct position. The plate 32 can be moved instead of the stylus 30.