If only one instrument, such as a millivoltmeter, is available, and there is no standard couple at hand, the new couple may be calibrated over a wide range of temperatures by the use of the following standards: Water, boiling point 212 deg.F. Tin, under charcoal, freezing point 450 deg.F. Lead, under charcoal, freezing point 621 deg.F. Zinc, under charcoal, freezing point 786 deg.F. Sulphur, boiling point 832 deg.F. Aluminum, under charcoal, freezing point 1,216 deg.F. Sodium chloride (salt), freezing point 1,474 deg.F. Potassium sulphate, freezing point 1,958 deg.F. A good practice is to make one pyrometer a standard; calibrate it frequently by the melting-point-of-salt method, and each morning check up every pyrometer in the works with the standard, making the necessary corrections to be used for the day's work. By pursuing this course systematically, the improved quality of the product will much more than compensate for the extra work. The purity of the substance affects its freezing or melting point. The melting point of common salt is given in one widely used handbook at 1,421 deg.F., although chemically pure sodium chloride melts at 1,474 deg.F. as shown above. A sufficient quantity for an extended period should be secured. Test the melting point with a pyrometer of known accuracy. Knowing this temperature it will be easy to calibrate other pyrometers. PLACING OF PYROMETERS.--When installing a pyrometer, care should be taken that it reaches directly to the point desired to be measured, that the cold junction is kept cold, and that the wires leading to the recording instrument are kept in good shape. The length of these lead wires have an effect; the longer they are, the lower the apparent temperature. When pyrometers placed in a number of furnaces are connected up in series, and a multiple switch is used for control, it becomes apparent that pyrometers could not be interchanged between furnaces near and far from the instrument without affecting the uniformity of product from each furnace. Calibration can best be done without disturbing the working pyrometer, by inserting the master instrument into each furnace separately, place it alongside the hot junction of the working pyrometer, and compare the reading given on the indicator connected with the multiple switch. Protection tubes should be replaced when cracked, as it is important that no foreign substance is allowed to freeze in the tube, so that the enclosed junction becomes a part of a solid mass joined in electrical contact with the outside protecting tube. Wires over the furnaces must be carefully inspected from time to time, as no true reading can be had on an instrument, if insulation is burned off and short circuits result. If the standard calibrating instrument used contains a dry battery, it should be examined from time to time to be sure it is in good condition.