1884.] New Spring for Electric and other Instruments. 317 



formed between D and E, which are the poles of the electromagnet. 

 To the iron tube AA, also made of charcoal iron, the spiral spring, in 

 this case made of extremely thin hard steel, is attached, the other end 

 being attached to the piece F, which is fixed relatively to the bobbin. 

 The spindle GG, which is fixed to the moving iron core AA, moves 

 freely in bearings HH, so that the only movements of which A is 

 capable are one of rotation and one parallel to the axis of the bobbin. 

 As the iron core A projects into the strong magnetic field between D 

 and E it is strongly attracted towards E when the current flows, and, 

 as before, causes a large rotation of the pointer P over the scale. As 

 a means of varying the power of the instrument an adjustable iron 

 piece K is provided, which can be screwed nearer to or farther from 

 the core A, and by the use of which the sensibility of the instrument 

 can be adjusted so as to make the instrument " direct reading," that 

 is to say, each division of the scale can be made to correspond with. 

 1 ampere of current, or 1 volt difference of potential, and the employ- 

 ment of a constant such as 1*34 amperes, or volts, per degree, which 

 has hitherto been necessary with our measuring instruments, is now 

 avoided. This power of adjustment produced by the use of the 

 movable iron piece K, combined with the ease with which more or 

 less wire can be wound on to the instrument, which also constitutes a 

 second adjustment of sensibility, is of considerable importance, since 

 the employment of a constant has not only led to error and delay in 

 measurements made in electric light factories, but has caused the 

 breakage of the pointer or the destruction of an instrument from a far 

 too powerful current being sent through it by an observer (often a 

 man with little experience in the employment of instruments) having 

 confounded the constant of some other instrument with that of the 

 one he was using. 



The steel strip used in the construction of the spring for these 

 shielded instruments is O'OOl/ 7 inch thick, and in a book just pub- 

 lished on " Steel and Iron," by Mr. Greenwood, he mentions that 

 some specimens of remarkably thin steel, t ±-q of an inch, were shown 

 in the Paris Exhibition, so that steel we are using is nearly as thin as 

 that referred to by Mr. Greenwood. But within the last few days we 

 have received from Mr. Charles Jowitt, of Sheffield, a specimen of steel 

 rolled for us only just over O'OOl inch in thickness, which is perhaps 

 the thinnest steel yet made . 



"We have to thank one of our assistants , Mr. Bower, for so earnestly 

 carrying out a long series of experiments on these very delicate springs. 



Weighing Machines. — Another class of instruments in which we 

 have practically employed this spring are weighing machines, and 

 fig. 9 shows one of the arrangements we adopt. The scale-pan is 

 prevented from turning by the part AB being square and fitting very 

 loosely a square hole in C. This arrangement introduces practically 



