532 Professor Howard T. Barnes [May 31, 



copper tube about 6 inches long and 4 inches in diameter. On 

 the surface of the inner tube the coil of resistance wire is wound. 

 The outer tube fits closely over this and the ends are soldered 

 together. This makes a winding which affords a large cooling 

 surface and is exceedingly sensitive to temperature changes. The 

 coil consists of 250 feet of large size iron wire silk-covered, and has a 

 resistance of approximately 125 ohms at 0° C. I used iron wire for 

 two reasons, its cheapness and its remarkable steadiness of zero 

 for ice temperature readings. The larger size wire enabled me to 

 apply considerable battery power without appreciable current heating. 

 My experiments with platinum were most unsatisfactory. There 

 was great current heating and changes of zero which as yet I have 

 not been able to explain. The iron wire maintains an invariable zero. 

 When iron is heated over 50° C, however, its zero does change and 

 can only be restored by cooling to a low temperature. It appears 

 that there must be some internal molecular change in the iron 

 above 50° C. which has not as yet been studied. 



The connecting wires from the bull) pass to a lead-covered cable 

 of four strands, as in Callendar's compensating method, and then to 

 the recorder, which may be a good Callendar recorder with the 

 galvanometer removed. The galvanometer actually used consists 

 of a jewelled-bearing D'Arsonval, of special design, of about 300 

 ohms resistance. The bridge wire may be interchanged so as to 

 produce different scales. The one I have found useful is 8 inches 

 long, and gives a range of one degree over its entire length. I have 

 used also one giving half a degree, and the records are very nearly 

 as perfect as with the one degree range. Other wires giving four 

 and eight degree intervals are frequently useful. By my arrange- 

 ment it is possible to switch from one wire to another very quickly. 

 The one degree scale is essential in very cold water, but for warmer 

 water the coarser scale is necessary. 



The thermometer may be connected so as to read the temperature 

 directly, in which case the resistance of the bulb is compensated by a 

 known resistance box. In this way the value of the resistance 

 from previous calibration gives a measure of the temperature. 

 The thermometer bulb may also be connected differentially with a 

 second equal bulb, and only differences in temperature recorded on 

 the chart. This method can be applied to give the difference 

 in temperature between the 1)0W and the stern of a ship. Thus 

 a ship so equipped could not possibly run into ice as long as the 

 bow thermometer was equal to the stern thermometer. 



In order to measure the temperature of the river or sea water 

 the thermometer bulb can be trailed by guy ropes alongside, and 

 the lead cable carried through heavy copper pipes up to the deck 

 of the ship. From there the cable can be carried to the chart- 

 house or other convenient location. The thermometer bulb may 

 also be placed in a tank fed from the circulating water in the engine 



