376 Mr. W. Petrie's Results of Experiments 



' 580*^-^' 



Q = 580 . S . 



( l 



580. S-L.?' 



<? = 580. S.' nn T — ~* 

 i 580.S+L.Q 



which appears to accord with some results arrived at by 

 Ohm, which I have subsequently seen ; though he has not 

 deduced formulae of quite the same nature. 



These formulae are useful to solve many practical questions, 

 such for instance as relating to the size of conducting wires, 

 and power of batteries necessary to transmit a given quantity of 

 electricity through a given distance, as in telegraphs ; and in 

 determining the ratio of the section to the length of helix 

 wires in forming electro-magnets of various dimensions and 

 proportions, so as to obtain a maximum of effect, which de- 

 pends chiefly on the number of coils x the quantity of elec- 

 tricity in circulation. 



The constant 580 in the formulas applies only to copper 

 wire unannealed ; to adapt the formulae to different metals 

 we must vary the constant in the direct ratio of their con- 

 ducting powers. The above calculations and experiments 

 refer only to electricity of the intensity of a single cell of 

 Daniell's constant battery. It is probable (judging from an 

 experiment with electricity of two cells' intensity) that an in- 

 crease in the intensity has the same effect on the transmission 

 of the electricity, as a proportionate increase in the conduct- 

 ing power of the wire, as Ohm had previously deduced. 



The ordinary form of galvanometer, by the deflection of the 

 needle in a helix placed so as to be in the magneCic meridian, 

 cannot determine the comparative amounts of two currents 

 of electricity by any calculation from the degrees of deflec- 

 tion, unless previous data be obtained by other experiments, 

 as the amount of deflective force exerted by any given cur- 

 rent at different arcs of deflection (when balanced by ter- 



* These formulae were tested by the following experiments : — A length 

 of copper wire (220 feet) was led about a room, double, that is, the re- 

 turning half running close and parallel to the other part of the wire with- 

 out touching : the doubled wire dipped into pairs of mercury cups at cer- 

 tain intervals ; any of these could be connected or disconnected in an in- 

 stant, by inserting or removing a bit of connecting wire, and thus the 

 current was transmitted through various lengths of wire successively. The 

 power of the battery was also varied by increasing the number of united 

 cells, or diminishing the strength of the acid. A magnet galvanometer, 

 included in the circuit, showed the quantity of electricity passing, cither 

 through the wire or without it. 



