376 PHYSICS. 



union of the first two coils with tlie axis which carries the alidade. This 

 arm is so placed that the galvanometer is at zero, and the reading is 

 noted. The two coils are then reversed in position, and a second reading 

 taken. The difference is the difference in their resistance. Were it not 

 for changes of temperature, measurements could be made to the yqoVo o 

 of an ohm. {Phil. Mag., V, ix, 109 ; J. Phys., March, 1881, x, 135.) 



Glazebrook has called attention to an error which results when small 

 resistances are measured by the Wheatstone's bridge, due to thermo- 

 electric forces which have their seat at the point of contact of the cop- 

 per and the platinum of the apparatus. The result is that the resist- 

 ances found seem to depend on the resistance of the battery. They are 

 eliminated by reversing the battery current. [Phil. Mag., April, 1881, 

 V, xi, 291; J. Phys., November, 1881, x, 500.) 



Minchin has given an account to the London Physical Society of his 

 new sine electrometer. It consists of two metal plates, in one of which 

 is an aperture nearly closed by a metal trap-door, suspended from the 

 plate by two fine platinum wires, and when the plates stand vertical, 

 resting against fine stops. These plates are connected to the poles of 

 the cell to be measured, and tilted out of the vertical till the attraction 

 of the whole plate on the suspended trap or shutter is just balanced by 

 the weight of the latter. Then the electromotive force is proportional 

 to the sine of the angle of displacement. [Nature, May, 1881, xxiv, 95.) 



Mascart has modified the quadrant electrometer of Thomson, reducing 

 materially its size, and adapting it to the ])urpose of meteorological regis- 

 tration by means of photography. The quadrants are kept charged by 

 a few cells of water-battery, and the water-dropping collector, used to 

 obtain the atmospheric j^otential, is in communication with the needle, 

 the case being connected with earth. The same photographic devices 

 have been emi^loyed to record magnetic variations, the horizontal force 

 beiiig given by a bifilar magnetometer, the vertical force by a magnetic 

 balance, and the declination by a declinometer, all the tracings being 

 obtained upon a single sheet of paper. ('/. Phys. June, 1881, x, 229.) 



Bailie has employed the torsion balance to measure electromotive 

 forces, and has obtained results agreeing well with those made by other 

 methods. {Ann. Ghim. Phys., June, 1881, Y, xxiii, 2G9.) 



Pellat has studied the discharge of a condenser by means of a tele- 

 phone. With reference to the current necessary to give an audible 

 sound in the telephone, he says: "Experiment has shown me that the 

 energy corresponding to a calory, that is, to the amount set free by a 

 gram of water cooled 1<^ C, transformed into electricity and sent through 

 the telephone, will produce a continuous and clearly perceptible sound 

 for ten thousand years!" {J. Phys., August, 1881, x, 358.) 



Edison has contrived several forms of an instrument for measuring 

 electrical currents, which he calls a " webermeter." In the form to be 



