METHODS AND APPARATUS. 17 



the whole moving system may easily be taken out and replaced by another, 

 perhaps of different resistance or sensitiveness; second, that small windows 

 are provided at the top and bottom of the suspension strip, by means of which 

 it may easily be inspected or removed. The chief disadvantage for general 

 work is, perhaps, that when the instrument is set up the moving coil is not 

 visible and, the clearance being small, leveling becomes comparatively difficult. 

 The resistance, 50 ohms, is rather too high for work of this character; prob- 

 ably a smaller resistance could have been used and still leave an instrument 

 of sufficient sensitiveness. The galvanometer is not entirely free from thermal 

 electromotive forces; indeed, this could hardly be the case where a number 

 of metals are in circuit. This disturbing effect has been found to be less, 

 however, in an instrument purchased in 1907 than in a later model purchased 

 in 1909. With a view toward reducing extraneous electromotive forces in 

 the galvanometer, it has been shielded as much as possible from temperature 

 changes by placing over it a cork-lined wooden box. This is provided with 

 a door at the side for inspection, leveling, etc., and with a transparent celluloid 

 window in front through which the rays of light can pass. Since the use of 

 a telescope for reading was regarded as too tiring for repeated observations, 

 a ground-glass scale at a distance of almost 4 meters has been used, upon 

 which the reflection of the filament of an incandescent lamp is focussed. If 

 too long a scale distance should be used, in the attempt to increase the sensi- 

 tiveness, a blurred image will result, caused by slight imperfections in the 

 surface of the mirror. 



The galvanometer has been mounted in the following way: A location 

 was chosen near a structural steel upright on the street floor of the building 

 and a strong shelf built out from the wall at this point. Upon the shelf was 

 placed a square block of cork, 3 or 4 cm. thick, to which was fastened a 1.25 

 centimeter iron plate, and on this the galvanometer was mounted. The very 

 slight vibration of the shelf resulting from the jar of the building is damped 

 out by the cork, which acts as a spring. 



The copper wires running to the thermal junctions have a diameter of 

 1.63 millimeters up to within 1 or 2 meters from the junction itself. At this 

 point they are joined to smaller copper wires (0.455 millimeter), thus making 

 the junctions smaller and more flexible. The wiring should be done in such a 

 way as to leave no avoidable loops in the circuit; this will eliminate the induc- 

 tive action of neighboring circuits, which otherwise might cause annoying 

 deflections of the galvanometer. 



THERMAL-JUNCTION SYSTEM. 



Thermometers for internal temperatures. Most of the junctions used for 

 measuring temperature in the natural and artificial cavities of the body are 

 simply twisted and soldered joints between two wires one of constantan 

 (0.455 millimeter) and the other of copper (0.455 millimeter). For taking 

 internal temperatures a construction like A (fig. 3) is employed, in which the 

 two wires run side by side, one of them being inclosed in a rubber tube for 



