Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

3APPARATUS9TerminalblockPlatinumresistancet herrnometerISlidingdoorsLooseasbestosInconeltubesilicatubeThermocouplewellI"S i ndan yo 'IFig. 5. A typical furnace.ture along the length of the furnace. Tappings are taken at regular intervals alongthe winding. The wire is covered with asbestos paper and cement and placed in anasbestos tube or box and packed into 3 in. of kieselguhr or other insulating material.Shunts are connected at the taps in the required places and adjusted until thetemperature profile at say 200" is f 0.5" along the length of the RV. The totalresistance of the furnace should be such as to allow a maximum output of fl Kwdepending on the temperature range to be covered. For photochemical or radiochemicalstudies the design has to be modified such that the furnace rests on anoptical bench and an uninterrupted path is provided for the source of radiation.This is achieved by splitting the furnace in half longitudinally or laterally. The latteris described in detail in the excellent treatise on photochemistry by Calvert andPithg. The former presents some difficulty because winding has to be carried outover a semicircular cross-section but can be achieved by drilling holes or havingpegs at regular intervals at the extremities of the asbestos tube halves. The inconeltube is replaced by two aluminium half-tubes with horizontal sections such thatone half sits on the other when they are slotted and clamped together. Entry tubesto the RV may be led in at the join of the two halves. This design is particularlyuseful for flash photolysis studies.A circuit for heating and controlling the temperature of a thermostat or a furnaceis shown in Fig. 6. Just the right amount of current from a constant voltagetransformer and a variac transformer is fed into the furnace connected in serieswith a small variable resistance N && of the value of the furnace resistance, connectedacross a relay. For thermostats, the relay may be operated by a toluene/mercury or a mercury thermo-regulator, depending on the temperature. Platinumresistance thermometers or thermocouples, forming part of a Wheatstone bridgenetwork, are used to operate the relay for electric furnaces. This type of regulatoris available commercially. A strict control of current is required. In this way it isReferences pp. 104-111