336 ON A NEW SELF-RECORDING THERMOMETER. 



What is required is an instrument which will accurately record 

 every change of temperature, and at first sight it seems that the 

 photographic method gives all that is desired. In this the rise 

 and fall of the mercury cuts off or increases the light which is 

 arranged to pass through the tube and fall upon a piece of 

 sensitive paper. The instrument works beautifully, but is subject 

 to the following objections : in order to get a column of mercury 

 large enough, the bulb has to be of considerable size, and therefore 

 simply from its mass cannot change its temperature as quickly as 

 it ought to do, and then the line of light which passes its tube 

 falls on the paper which is moving slowly forward, so that slight 

 oscillations overlap upon the paper and are thus obliterated. For 

 instance, suppose there is a change of half a degree in one minute 

 of time, the line of light covers more space than the paper moves 

 in a minute, so that although the mercury has risen and cut off 

 the light from a part of the paper equal to half a degree in height, 

 the clock has not moved it on fast enough to bring a new part of 

 the paper under the new condition wholly, but the part presented 

 to the light at the end of the minute is nearly the same as it was 

 at the beginning, and hence the light acts upon it, and in these 

 two ways all the sudden changes of temperature get lost. To 

 a great extent this must always be so when glass thermometers 

 are used, because if large enough to record they take some time 

 to shew changes of temperature, as every one knows who has had 

 anything to do with them. 



The plan I have adopted, I have had at work for some weeks 

 in a very rough model, which I have brought here to shew you, 

 as I think it is quite new. It was suggested to me as an 

 application of a principle of recording small changes, which I 

 recently brought before another scientific society as a means of 

 recording changes in the " direction of the vertical" and it may be 

 briefly described as the method of electric contact. This method 

 affords a means of testing minute changes of position with 

 surprising accuracy ; one thousandth part of an inch is a small 

 quantity, and a tenth of that is a quantity that few processes in 

 in the arts will deal with ; but the method I am using will detect 

 with certainty, even in this rough model, one-third of that small 

 quantity, or ^oloo part of an inch, and will, I am sure, when 

 properly made detect T o 0V0 it part of an inch of change. 



In the model you will observe that I havea cylinder 26 inches long 

 and 10 inches in circumference, (see diagram) and that over this an 

 electrical pen is worked gradually along as the cylinder turns, it 

 traverses 24 inches in the day or 1 inch per hour ; fixed to the 

 end of the cylinder as you see, is a wheel with teeth only half way 

 round it, along side of this is a very carefully made screw with 50 

 threads to the inch, and on this screw a wheel with as many teeth 





