C. GENERAL PHYSICS. 167 



liave to resist shocks, should have large factors of safety, and 

 be protected, if possible, from extremes of temperature. 



THE EFFECT OF A GALVANIC CURRENT ON THE ELASTICITY 

 AND LENGTH OF THE CONDUCTING WIRE. 



As the result of a very complete investigation by Streintz 

 into the effect of a galvanic current on the elasticity and 

 length of the wire through which it is passing, the author 

 oives the following conclusions : 



1. The current does effect a change in the elasticity of its 

 conducting wire, but only by virtue of the heat that is there- 

 by developed. 



2. The current extends the conducting wire more than it 

 would be extended by simply warming up to the same tem- 

 perature. Such excess of extension is inappreciable only in 

 the case of hard steel. 



3. The galvanic extension is effected, not suddenly, in con- 

 sequence of the completion of the current, but gradually, in 

 a manner similar to the effect produced by heat. 



4. The galvanic extension can not be a consequence of an 

 electro-dynamic repulsion, but probably consists of a polari- 

 zation of the heat vibrations. Sitz.JC. K. Akad. der Wiss., 

 Wien, LXVIL, 354. 



THE HORIZONTAL PENDULUM. 



Zollner describes a series of experiments, with a form of 

 horizontal pendulum, of such surprising delicacy that it opens 

 a wide field for investigation. It has, indeed, been shown 

 that the instrument suggested by Zollner was described, and 

 its uses suggested, in the early part of the present century, 

 by Gruithuisen, whose account, however, had quite fallen into 

 oblivion until Zollner's reinvention of the same delicate in- 

 strument. The apparatus consists of a short horizontal lever, 

 suspended from a fixed point A by a vertical piece of fine 

 watch-spring attached near one end of the lever, and carry- 

 ing at the long end a heavy leaden weight and mirror. The 

 short end is prevented from rising by a second watch-spring 

 fastened to the ground below at a point B. The two points 

 A and B are nearly vertically above each other, and are 

 equidistant from the lever; they may be so adjusted that 

 the pendulum will assume a position of equilibrium, around 



