284 PHYSICS. 



by tapping- the ground at places of different potential, the author made 

 experiiueuts to determine the equipotential surfaces formed by the 

 ground of the time signal-service. By running a wire 500 or GOO feet, 

 placing its terminals in moist earth, and including a telephone in the 

 circuit, the ticking of the clock could be readily heard in a field an 

 eighth of a mile distant from the observatory ; and at one point a mile 

 distant the ticking was heard with only 50 feet of wire. Behind the 

 observatory no result was had, though at no point were earth currents 

 absent. " Tlicoretically," the autiior concludes, " it is possible to day 

 to telegraph across the Atlantic Ocean without a cable" ; "practically 

 the expenditure of energy on the dynamo-electric engines would seem 

 to be enormous." — {Am. J. Sci., Ill, xx, 138, August, 1880.) 



Hill has devised an electro-d;\namometer for use especially with strong 

 currents, which in general is similar to Trowbridge's instrument, but 

 differs from it in the manner of determining the deflective power of the 

 current. To the suspended coil a pointer-rod is attached, on opposite 

 sides of which silk threads are fastened, which lead over pulleys on the 

 side bars to small pans, one on each side of the instrument. When the 

 deflection has taken place weights are added to the pan on the opposite 

 side until the pointer-rod returns to zero. In the instrument as con- 

 structed a current of 20 webers required about half a gram weight, and 

 one of 100 webers a weight of 13.2 grams. The instrument works well 

 and gives uniform results. — {Am. J. /S'ci., xix, 10, January, 1880.) 



Hughes has devised an apparatus, which he calls an induction balance, 

 for comparing induced currents. It consists (1) of a battery of three 

 Daniell cells, the current of which traverses a microphone with an at- 

 tached clock for producing the sounds; (2) of the balance proper; (3) of 

 an electric sonometer ; and (4) of a receiving telephone. The balance 

 itself is formed of four equal coils, a decimeter in height, 5.5<=™ external 

 diameter, with a space of 3^"' internally, each covered with 100 meters of 

 No. 32 covered copper wire. The four coils form two groups placed be- 

 yond each other's influence. At each end, therefore, there are two 

 coils, one inducing, the other receiving the induction, jdaced end to 

 end, and about 5'"™ apart. The electric sonometer is formed of two sim- 

 ilar inducing bobbins fixed at the ends of a horizontal rule 40°"^ long, 

 divided into millimeters. Along this rule moves a receiving coil. The 

 electric current from the microphone reaches a commutator which directs 

 it into the balance or into the sonometer, where it traverses the induc- 

 ing coils. The telephone circuit passes through the receiving coils, first 

 the two on the balance, in opposite directions, and then that on the 

 sonometer. No sound is heard in the telephone if the receiving coil in 

 the latter case be equidistant from the inducing ones. No sound is heard 

 in the balance coils if they are empty ; but if a fragment of metal be 

 placed between them, a sound is heard louder according to the character 

 of the metal. The current is passed alternately into the balance and 

 the sonometer, and the coil of the latter is displaced until the sounds 



