This mean temperature difference is in practice usually spoken of as the "temperature head" or "heat drop." It will be clear that this temperature head is increased by using steam at higher pressure (temperature), and by evaporating under reduced pressure. Since most liquids have their boiling points reduced about 40° C. by operating _in vacuo_, the advantage of the vacuum is apparent. It should be remembered that the temperature head has not the same value in any part of the scale: it has more value higher up the scale, because the steam is denser and more heat units come in contact with a given area in a given time. It must also be remembered that whilst the pressure gauge is a most useful indicator of steam temperature, it is not necessarily accurate. The pressure in the hot space is the _sum_ of the pressures of air and steam, and since the temperature (the important condition) of the hot space depends upon the pressure of the _steam_, and not on the sum of the pressures, the temperature in a steam space is always rather lower than would be supposed from the pressure indicated by the gauge. The transference of heat is influenced by the velocity of both the heating fluid and the fluid being heated over the heating surface. The more rapidly each fluid moves, the more rapid is the transference of heat, because a greater number of particles of both fluids are brought to the heating surface in any given time. This is popularly known as the effect of "circulation," and is illustrated by the advantage of stirring a liquid being heated in bulk. In the film evaporators the circulation is through tubes at high speed (up to 2 miles a minute), and the maximum effect in this sense is thus obtained. The increase in heat transference is not directly proportional to the increase in velocity, but in a lower ratio, sometimes approximately the square root of the velocity. In such a case, if either velocity be quadrupled, the heat transference is doubled. Other advantages of high velocity are that the heating steam more readily sweeps away condensed steam from the heating surface, and the high-speed film similarly "scours" away "incrustations" on the interior of the tubes. The transference of heat is also proportional to the conductivity of the metal forming the heating surface. For gelatine liquors, copper tubes are almost invariably employed, the advantage being great even when price is taken into consideration. The following conductivity coefficients illustrate this point (calories per hour through 1 sq. metre of metal 1 metre thick, with a temperature difference of 1° C.):-- Copper...330 Iron.....56 Steel....22-40 Tin......54 Zinc.....105 Lead.....28 The coefficient of heat transmission decreases the more with increasing thickness of wall, the worse conductor is the metal. For copper tubes, however, this decrease is usually unimportant.