

UNDULATORY FORCES. -ELECTRO-MAGNETISM. 



[TELBORmiS. 



read in? thorn. These two desiderata have been accom- 

 plished by Mr. Wheatstone. The instrument consists of 

 two parts the communicator and indicator on each of 

 which the letter* of the alphabet, and numerate are 

 engraved, in the form of a due. Ity means of a needle, 

 actuated by electro-magnetic machinery, the words of t ho 

 menage, sent by the communicator are spelt, letter by 

 letter, on the indicator ; the former instrument being in 

 possession of the sender, and the Utter with the receiver ; 

 the current is conveyed by wires, suspended in the air 

 in one bundle or rope, which may be made to enclose 

 almost any number, from their small sectional area. 

 Round the discs of both instruments, as many keys are 

 placed as there are letters ; and being pressed down, as 

 each letter required is arrived at, in the rotation of the 

 needle, they indicate the letter which has been tele- 

 graphed. The electric power Li obtained by a modifica- 

 t : m of the magneto-electric machine that is, by magnetic 

 induction ; and thus, the expense and nuisance of a 

 voltaic battery are obviated. The preceding engravings 

 will give a general idea of the instrument. 



Special instructions are given as to the arrangement of 

 the apparatus before sending a message ; and the earth 

 is used for the return current, as in the ordinary method 

 of telegraphy. 



The instruments, and use of tho wires, are charged for 

 by an annual rental. And thus, any two individuals, 

 bating agreed to employ the apparatus, and having had 

 it suitably fixed, may cany on a correspondence, either 

 in letter or cipher, entirely between themselves, and 

 without the possibility of their secrecy being invaded by 

 curious telegraph clerks, or interested parties. This is, 

 of course, ensured by the use of separate and isolated 

 eta of wires for each sot of telegraph instruments. 



Such inventions as these cannot fail to be of great 

 benefit, in a social, commercial, and political point of 

 view. In great cities, it is of extreme importance that 

 police, military, and fire-engine stations, should have 

 instantaneous means of communication ; and tlic.su are 

 at once afforded by the private telegraph system. 



BALLOOX TELEOKAPHS. During the long wars of 

 France, Napoleon conceived the happy idea of employing 

 balloons for the purpose of surveying the enemy's lines ; 

 and the same plan was adopted by the Federals during the 

 unfortunate internecine struggle between tho American 

 States in 1862. A most important addition, however, was 

 made in the application of the electric telegraph. The 

 balloon was connected by means of wires with the earth, 

 and thus a continued communication was kept up be- 

 tween the observers and their attendants on terra Jirma. 

 By this ingenious plan most valuable information was 

 obtained, and the course of the campaign materially 

 influenced. 



SIEMENS' ELECTRIC RESISTANCE THERMOMETER. This 

 peculiar arrangement, primarily intended to test the 

 temperature of large coils of submarine telegraph wires, 

 in cafes where spontaneous heating was feared, is an 

 adapti tiou of the laws of resistance in conducting-wires, 



the resistance being measured by moms of an ordinary 

 galvanometer. It is equally applicable to tho disc 

 of heating in masses of hemp, SO. , which have frequently 

 caused disastrous conflagrations. We are indebted to 

 the London lieview for a description of its arrangement. 



Tho apparatus consists of a differential galvanometer 

 and of a bath of water (or oil), tho temperature of which 

 can bo changed at will by opening one or the other of two 

 cocks, tho one bringing a supply of cold, and the oil 

 supply of hot liquid, an overflow pipo being provided to 

 prevent accumulation. A battery of from four to eight 

 cells is also provided, besides a number of electric a 

 consisting each of a certain length of thin insulate 1 

 platinum wire inclosed in a sealed metal tubo. 



These coils having been carefully adjusted, in the first 

 instance, so as to oiler equal resistance at one temperature, 

 are connected to insulated copper leading wires of 

 parativoly largo sectional area (No. 1(>, B. \V. G.), tho 

 ends of which are brought to tho binding screws of tho 

 apparatus, to bo inserted, when required, into a circuit 

 including the battery and tho galvanometer. 'i 

 " thermometer coils" are deposited at the places wliero 

 the temperature is to be observed, excepting one which 

 has to bo reserved for comparison with the others. This 

 last-mentioned coil is, through its leading wires, so con- 

 nected as to form an clectrio circuit with th ; b . 

 and the second coil of the differential galvanometer, ;u: I 

 is immersed in the bath before mentioned. It is evident:, 

 that if the temperature of the bath should be the samo 

 as that of the place where tho thermometer coil u> 

 examination is deposited, the two electric currents pro- 

 ceeding from tho battery will meet with > 

 in the two circuits, and, in passing two spirals of the 

 differential galvanometer in opposite directions, will pro- 

 duce no visible effect upon the needle. If, however, tlie 

 temperatures of thermometer and balancing coil should 

 be unequal, the needle will be deflected by the pi 

 dorance of current in the cooler circuit, showing, by the 

 direction of its deflection, whether cold or hot wain- 

 should be added to tho bath to establish equilibrium of 

 currents. When this equilibrium is obtained, the tem- 

 perature of the bath is observed by means of an ordinary 

 mercury thermometer, which temperature must be iden- 

 tical with the temperature at tho distant place. 



In dividing the " thermometer coils" into two por- 

 tions, the apparatus is rendered applicable for observing 

 wider ranges of temperature than can be directly attained 

 by the mercury thermometer, and in this modified form 

 it may be used for pyrometrical purposes. In measuring 

 ordinary temperatures, the employment of equal and 

 undivided coils is, however, not only tho most simple 

 arrangement, but it has the advantage in its favour, that 

 the accuracy of tho observation is not dependent upon 

 the rate of variation if resistance bo uniform, or oven 

 accurately determined. The heat produced in tho coils 

 by tho electric currents employed, affecting the two coils 

 equally, is also completely compensated in using 

 coils. 



