422 Merritt — Peculiarities in Behavior of a Galvanometer. 



care to avoid as far as possible all sources of error, and obtain 

 the following results as the mean of a number of measure- 

 ments : 



First throw. Final deflection. Eatio. 



53-0 mm . 196-0 mm . 0-270 



36-5 136-7 265 



17-75 65-0 0-273 



5 25 19-75 0-265 



The slight irregular variations which occur might easily be 

 accounted for by the unavoidable errors of observation, 

 especially as the source of heat, an Argand burner, could not 

 be relied upon as perfectly constant. The conclusion that the 

 ratio is independent of the deflection, seems therefore to be 

 justified, at least in the case of the galvanometer used in these 

 experiments. 



The importance of this conclusion will readily be seen by 

 those who have had occasion to use a thermopile for accurate 

 measurements of radiant heat. Draughts of air, and other 

 almost unavoidable sources of temperature variation, frequently 

 make the galvanometer quite unsteady, while the extreme 

 delicacy of the instruments that must be used in work of this 

 kind renders them especially susceptible to magnetic disturb- 

 ances. Many observations are thus made valueless by a change 

 in the zero point of the galvanometer during the three or four 

 minutes required for the needle to come to rest. Only a few 

 seconds are required, however, for the first throw of the 

 needle, and the change in zero point during this time would 

 scarcely ever be sufficient to cause an appreciable error. The 

 use of the first throw in place of the final deflection may 

 therefore lead to greater accuracy as well as to a saving of 

 time. 



It will be observed that the- principle underlying this method 

 of taking readings is not confined to the case of the thermopile, 

 but is capable of quite wide application. The following are 

 suggested as cases in which the method may be employed with 

 especial advantage : 



(1) All ordinary measurements of radiant heat, when the 

 conditions are such as to make the galvanometer unsteady, or 

 when the saving of time is a consideration. 



(2) For purposes of demonstration in the lecture room. A 

 number of experiments which are usually considered unavail- 

 able for lecture demonstrations have been quickly and accu- 

 rately performed in this way under conditions that would render 

 the use of the ordinary method entirely out of the question. 



(3) For measurements of the heat from a variable source. 

 The first throw of the needle will in this case give the amount 

 of radiant energy at the very instant of exposing the pile. 



