ICEBERGS IN NAVIGATION BARNES. 



725 



ideas which I had developed along these hues, I devised an instru- 

 ment which has proved so satisfactory and trustworth}' that I have 

 given it the name of the microthermometer. 



The thermometer bulb (fig. 2) consists of concentric cylinders of 

 copper tube about 6 mches long and 4 mches hi 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 wliich affords a large cooling 

 surface and is exceedingly sensitive to temperature changes. The 

 coil consists of 250 feet of large size iron %\are, 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 readmgs. The larger size wire enabled me to 

 apply considerable battery power without appreciable current heating. 

 ^ly experiments with platinum were most unsatisfactory. There 



Fig. 2.— Bulb of micro-thermometer. 



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 coohng to a low temperature. It appears 

 that there must be some internal molecular change in the iron 

 above 50° C, wliich has not as yet been studied. 



The connecting wires from the bulb 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 witli the 

 galvanometer removed. The galvanometer actually used consists 

 of a jcwcled-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 1° over its entire length. I have used 

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

 perfect as with the 1° range. Other wires giving 4° and S° intervals 

 85360°— SM 1912 47 



