Feb. 16, 1883.] 



• KNOWLEDGE • 



103 



then substituting and modifying an unknown resistance 

 until tlie reduced deflection is repeated, we can obtain a 

 second resistance equal to tlie first, ^^'ith these two known 

 resistances, a third can be made eiiual to their combined 

 resistance, trnd, with a little thought, all the required coils 

 may be measured ofV. 



A third method is by that known as the Wheatstone 

 "bridge." This is of all methods the most universally 

 applicable and satisfactory. By its means we are able to 

 measure accurately and easily any resistance varying from 

 a minute fraction of an Ohm to several million Ohms. 

 There is very little new concerning the arrangement, but 

 its interest and importance are of so vital a nature in all 

 systems of measurement, that we fear no excuse we could 

 urge would be sufficient to warrant our passing it by. 



In Fig. 2, which illustrates the principle involved, the 



pole C of the battery is put to earth, or joined to the 

 other earth-wire from K. A wire passes from Z to a 

 terminal at H, where the current is divided, one part 

 going through A, M, D, and the other through B, N, E to 

 K, and thence flowing to earth or back to the battery. 

 G is a galvanometer joined to the two wires from 

 the battery at M and N. Ignoring for a moment 

 the galvanometer circuit (M G N), it will be remembered 

 that when a current is divided between two conductors, 

 the proportion passing through them is inversely as 

 their resistances. Furthermore, the electrical principle 

 determining the current's How is its potential or the 

 intensity of its electrical charge. A current from C to 

 earth or through K to Z is, therefore, an effort to establish 

 electrical eiiuilibrium thronghout the circuit, and is the 

 result of differences of potential in the various parts of the 

 circuit When a current thus flows, it is clearly passing 

 from a high to a low, or may be to Zero potential (the 

 nominal condition of the earth). In Fig. 3, Z C is a 



battery with Z to earth and C joined to the uniform wire 

 C B, the end B being to earth. The vertical line C A 



may be made to represent the potential or electric 

 charge at C, B (to earth) being Zero. Then the 

 inclined line A B depicts the gradual fall of potential 

 which takes place as the current passes along the 

 wire. It is apparent that as D L is half the length of 

 C A, the fall between C and D is equal to the fall between 

 D and B. Ne.xt suppose we have a wire E F similar to 

 C B, but twice the length. If we connect E to the C pole 

 of the buttery, the current will divide between the two 

 wires, but the current in each wire will have the same 

 electro-motive force (which, it must not be forgotten, is 

 the dill'erence of potential between the two poles of the 

 battery). The vertical line E G must therefore be equal 

 to C A, and E F being uniform there will be a gradual 

 fall of potential from E through F to eartlu If we draw 

 from the centre H of the line E F a vertical line II K, that 

 line will indicate that the fall of potential from E to H 

 is the same as that from 11 to F. 11 K is, in fact, half the 

 length of E G, and as it is also equal to D L, it implies that 

 the fall between C D is equal to that between E H, and the 

 fall between D B equal to the fall between H F. Let us now 

 join D to H. It is clear that these two points are at the 

 same potential — consequently, a current being the result of 

 a difference of potential, there will be no current flowing 

 through D H. If, on the other hand, the wire from D 

 be joined to any part of E F other than H, there will be 

 a current flowing. If joined to L, the current will flow 

 from L to D, because L is at a higher potential than D. 

 If joined to M there will be a current from D to M, 

 because the potential of D is higher than that of M. This 

 brings us back to Fig. 2, where M N is the wire joined to 

 the two lines through which the battery current is flowing. 

 G is a galvanometer with many turns of fine wire. If the 

 resistance blocks A B D E are all equal — say ten Ohms each 

 — the galvanometer needle remains steady, because the 

 potentials of the points M and N are equal. 



As we have already reached the limit of our space, we 

 must defer a further consideration of this most interesting 

 topic till our ne.xt article. 



COLLISIONS AT SEA DURING FOG. 



By Richard A. Pkoctor. 



THE care with which money is saved over the signal- 

 lamps of ships, ic. — especially steamships at night — 

 is most praiseworthy. Several pounds a year must be 

 saved by using only two side lights — the red port light and 

 the green light on the starboard side ; and what if, when 

 a collision occurs, half-a-million of money's worth may go 

 in a few minutes to the bottom ? A ship lost is a ship lost, 

 but economy must Vje attended to as all-important. 



But, seriously, when we consider the power which science 

 has attained over the forces of nature, when we remember 

 how, by virtue of such power, we can make our ships in- 

 dependent of wind and tide, and how we urge them — each 

 with its freight of living souls — along the great trade routes 

 in hundreds, it does seem to imply stupidity and want of 

 care, to say nothing worse, that such means as science 

 affords for defending them against the risk of collision 

 should not be employed. I will not here consider the 

 general question of collisions at sea, though it is one about 

 which much might be said. But such disastrous collisions 

 as have occurred recently might easily be avoided, or at 

 least rendered extremely infrequent, if proper means were 

 used for indicating, by strong lights suitably placed, the 

 position of steamships, whether in motion or at rest. 

 Everyone who has traversed a thick fog, even a London 



