ELECTRIC-RESISTANCE THERMOMETERS FOR THE WATER-CURRENT.

The measurement of the temperature differences of the water-current by the electric-resistance thermometer was tried a number of years ago by Rosa,[10] but the results were not invariably satisfactory and in all the subsequent experimenting the resistance thermometer could not be used with satisfaction. More recently, plans were made to incorporate some of the results of the rapidly accumulating experience in the use of resistance thermometers and consequently an electric-resistance thermometer was devised to meet the conditions of experimentation with the respiration calorimeter by Dr. E. F. Northrup, of the Leeds & Northrup Company, of Philadelphia. The conditions to be met were that the thermometers should take rapidly the temperature of the ingoing and outcoming water and that the fluctuations in temperature difference as measured by the resistance thermometers should be controlled for calibration purposes by the differences in temperature of the mercurial thermometers.

Fig. 16.—Details of resistance thermometers for water-circuit. Upper part of figure shows a sketch of the outside of the hard-rubber case. In lower part is a section showing interior construction. Flattened lead tube wound about central brass tube contains the resistance wire. A is enlarged part of the case forming a chamber for the mercury bulb. Arrows indicate direction of flow on resistance thermometer for ingoing water.

For the resistance thermometer, Dr. Northrup has used, instead of copper, pure nickel wire, which has a much higher resistance and thus enables a much greater total resistance to be inclosed in a given space. The insulated nickel wire is wound in a flattened spiral and then passed through a thin lead tube flattened somewhat. This lead tube is then wound around a central core and the flattened portions attached at such an angle that the water passing through the tubes has a tendency to be directed away from the center and against the outer wall, thus insuring a mixing of the water. Space is left for the insertion of the mercurial thermometer. With the thermometer for the ingoing water, it was found necessary to extend the bulb somewhat beyond the resistance coil, so that the water might be thoroughly mixed before reaching the bulb and thus insure a steady temperature. Thus it was found necessary to enlarge the chamber A (fig. 16) somewhat and the tube leading out of the thermometer, so that the bulb of the thermometer itself could be placed almost directly at the opening of the exit tube. Under these conditions perfect mixing of water and constancy of temperature were obtained.

In the case of the thermometer which measured the outcoming water, the difficulty was not so great, as the outcoming water is somewhat nearer the temperature of the chamber, and the water as it leaves the thermometer passes first over the mercurial thermometer and then over the resistance thermometer. By means of a long series of tests it was found possible to adjust these resistance thermometers so that the variations in resistance were in direct proportion to the temperature changes noted on the mercurial thermometers. Obviously, these differences in resistance of the two thermometers can be measured directly with the Wheatstone bridge, but, what is more satisfactory, they are measured and recorded directly on a special type of automatic recorder described beyond.