GENERAL METHODS OF INVESTIGATION, BY S. STRICKER. XV 



steadiness, if fed with regularity. It can be left closed for 

 several days, and yet the temperature of the tin-foil kept to 

 a definite degree, not Varying with that of the room. It seldom 

 requires water, but crystals of copper must be supplied at least 

 once a day, so that the solution may be constantly and equally 

 saturated. If we overlook, however, these slight drawbacks, 

 and reflect that such precautionary measures are only requisite 

 in cases where it is wished to maintain a particular prepara- 

 tion at a uniform temperature for many days and nights, we 

 shall feel that in the interest of such important investiga- 

 tions it can scarcely be thought too great a trouble to attend 

 at least once a day to the requirements of the machine. 

 If the amount of work performed by the battery be but small, 

 or if it be only occasionally applied, it will then long retain 

 its activity without requiring other addition than that of a 

 little water from time to time to supply the place of that 

 rhich is lost by evaporation. 



Meidinger's arrangement gives off no injurious vapours, and 

 lay therefore be enclosed in a little box, and placed beneath 

 >r near the work-table. I transmit the conducting wires through 

 loles bored in the table, and when required for use, fasten them 

 to the points indicated by + and in fig. v. 



Inasmuch as the temperature of a thin wire introduced into 

 a thicker arc is inversely as the square of this wire, whilst its 

 length, when small, is of no importance, it follows that the 

 method of measurement formerly employed is justified. But 

 it is also clear that the active force present can be accurately 

 accommodated on the basis of this law. For if the temperature 

 diminishes as the square of the strength of the current, this 

 decrease can, to a certain extent, be covered by diminishing the 

 transverse section of the tin-foil, so that if a weak current be 

 in use, the strip of tin-foil must be made proportionably narrow. 

 As these strips are easily torn, I am accustomed to glue the 

 tin-foil upon thin paper, and then cut out a very long strip 

 with its central window. The larger portion of the strip I 

 twine round the bulb of a thermometer in such a way that 

 after making several coils, the two ends hang free. I then, 

 cover the whole bulb with a layer of shellac or glass cement, and 

 pass it through a cork into an empty vessel, so that the ends of 



