162 ANNTAL OF SCreXTIFIC DISCOVERT. 



a rod of iron as a case. A wire, soldered to the coil, comes out 

 at riglit anij^lcs IVoin it; and, bcinij l)ont down at tlie end, dips 

 into a cup contaiiiirig mercury. The batter}' connection is so 

 arranged, that when the dipping wire is in the cup the galvanic 

 circuit is closed. On th(^ closing of the circuit, the dippi'r is 

 drawn out of the cup, and tiic circuit is thereby broken ; and tiie 

 coil, under the action of its electricity, returns to its former j)osi- 

 tion. The dipper is thus alternately lifted up, and plunged into 

 the cup, and a rapid series of interruptions is made. This inter- 

 ruption admits of a simjile and perlVnit system of regulation, so 

 that it can l^e made to move at any speed. It does away with 

 the armature and spring of ordinary self-acting brakes, is quite 

 continuous;, and introduces almost no resistance into the primary 

 circuit. Tiie interruption was used with the coil, and several 

 experiments illustrative of the merits of botli were performed. 



ELECTRIC SIGNALS ON RAn.ROADS. 



Before the Institution of Civil Engineers, an interesting paper 

 was read l)y W. H. Treece, Associate, "On tiie best means of 

 conununieating between the passengers, guards, and di'ivers of 

 trains in motion." Mr. Preece exi)lainod tliat tlie essentia! ]mn- 

 ciple of the system he had introduced in the trains of the South 

 Western, the Midland, and the tlreat Northern, was the exten- 

 sion of a single isolated wire throughout tlie wiiol*; train, wliich 

 was maintained in a state of electrical equilil)rium by having the 

 similar poles of every battery in each van and engine attached to 

 it, while tlieir opposite poles were connected witli the earth, so 

 that when this Cijuilibrium was disturljed by placing the wire to 

 earth tlu'ougli the framework, wlieels, and rails in any carriage 

 or van, the current from each battery acted upon the l)e]l in its 

 own van, and upon a signal on the engine. Its i^eculiarity con- 

 sisted in tliis, that the commutators in each compartment of every 

 carriage Avere protected from the miscliievous and idle I)y being 

 covered with glass, which had been found experimentally to be 

 tlie best material for the purpose, as any opaque substance excited 

 inquisitiveuess and interference. 



MUTUAL ACTION OF ELEMENTS OF ELECTRIC CURRENTS. 



The law of Ampere — that commonly recognized as expressing 

 the action of two independent elements (or infinitesimal portions 

 of electric currents) — was based on a certain assumption, and on 

 four cases of equilibrium experimentally determined. The as- 

 sumption of Ampere was, that the direction of the action was 

 constant, independent of the X'clative direction of the elements, 

 and that the quantity or intensity of the action varied with the 

 direction of the elements. The new assumption is the reverse of 

 this, to w-it, that the intensity of the resultant is independent of 

 the direction of the elements ; but that the direction of tlie result- 

 ant varies with that of the elements. The latter view moi-e closley 



