ON THE BIFILAR MAGNETOMETER. 269 



duces no inconvenience in its use, which would be the case to 

 a certain extent with the other magnetometer. It does not fol- 

 low that a bar of exactly five-and-twenty-pounds' weight, such 

 as we have used, must be employed ; one of ten pounds will 

 suffice for the most dehcate measurements, and even one of four 

 pounds might answer. The small bars have only one advantage 

 over larger ones, in the greater facility of imparting to them a 

 strong degree of magnetism; and this is only of importance 

 where powerful means of exciting magnetism by friction are 

 wanting. 



With respect to a suitable locality, a room similar to that em- 

 ployed for the unifilar is all that is requisite, even if a bar of 25lbs. 

 is employed. The breadth of the room may even be less, and its 

 length may form any angle with the magnetic meridian, because 

 the mirror in this case is not attached to the extremity of the mag- 

 netic bar, but to the stirrup at the centre of the bar, and it can 

 be turned in any direction. A considerable height is requisite, 

 so that the interval of the two threads or wires to which the in- 

 strument is suspended may be sufficient for convenient measure- 

 ment A\ithout rendering the directive force too great. As it is 

 rare that a room is sufficiently high, it is advisable to pierce the 

 ceiling, and to caiTy the wires as high as the roof wiU allow. 

 In regard to the height, it is of little consequence whether a 

 heavy or hght bar be placed in the stirrujD, supposing only both 

 bars to be proportionally magnetized, and both to be much 

 heavier than the stirrup. It is not necessaiy to construct a 

 separate building free from iron for the bifilar, as is done for 

 the other magnetometer ; it may be placed, as is the case at 

 Gottingen, in the middle of a room in a building from Avhich 

 iron has not been excluded: it is sufficient to remove all iron 

 from the immediate neighbourhood cTf the instrument : it is best, 

 however, to place it in the magnetic observatory which contains 

 the other magnetometer, if the room is large enough and adapt- 

 ed for the purpose. If, for instance, the changes of the declina- 

 tion and of the intensity are to be observed simultaneously du- 

 ring the terms, a double number of observers is necessaiy if 

 the apparatus are in different buildings. But if both are in one 

 large room, and so arranged that, whilst the magnetometers are 

 at a sufficient distance asunder, the theodolites with which the 

 observations are made are situated near one another, one clock 

 may serve both observers, and one practised observer may ob- 

 sen'c alternately with both instruments, allowing an interval 



