The cold junction of a thermo-couple may be connected by means of copper wires to the voltmeter, although in some installations of base metal couples, the wires forming the couple are themselves extended to the voltmeter, making copper connections unnecessary. From the foregoing, it may be seen that accurately to measure the temperature of the hot end of a thermo-couple, we _must know the temperature of the cold end_, as it is the _difference_ in the temperatures that determines the voltmeter readings. This is absolutely essential for precision, and its importance cannot be over-emphasized. When pyrometers are used in daily operation, they should be checked or calibrated two or three times a month, or even every week. Where there are many in use, it is good practice to have a master pyrometer of a rare metal couple, which is used only for checking up the others. The master pyrometer, after calibrating against the melting points of various substances, will have a calibration chart which should be used in the checking operation. It is customary now to send a rare metal couple to the Bureau of Standards at Washington, where it is very carefully calibrated for a nominal charge, and returned with the voltmeter readings of a series of temperatures covering practically the whole range of the couple. This couple is then used only for checking those in daily use. Pyrometer couples are more or less expensive, and should be cared far when in use. The wires of the couple should be insulated from each other by fireclay leads or tubes, and it is well to encase them in a fireclay, porcelain, or quartz tube to keep out the furnace gases, which in time destroy the hot junction. This tube of fireclay, or porcelain, etc., should be protected against breakage by an iron or nichrome tube, plugged or welded at the hot end. These simple precautions will prolong the life of a couple and maintain its precision longer. Sometimes erroneous temperatures are recorded because the "cold end" of the couple is too near the furnace and gets hot. This always causes a temperature reading lower than the actual, and should be guarded against. It is well to keep the cold end cool with water, a wet cloth, or by placing it where coal air will circulate around it. Best of all, is to have the cold junction in a box, together with a thermometer, so that its temperature may definitely be known. If this temperature should rise 20 deg.F. on a hot day, a correction of 20 deg.F. should be added to the pyrometer reading, and so on. In the most up-to-date installations, this cold junction compensation is taken care of automatically, a fact which indicates its importance.