466 Prof. R. Threlfall on the Measurement 



riment in which they were employed in the usual manner. 

 They were therefore roughly mounted, and provided with an 

 astatic combination of the kind previously described. The 

 results were so encouraging that the same methods were pur- 

 sued further. In the final form the suspension was a quartz 

 fibre, 85 centim. long, suspended in a carefully chosen glass 

 tube. With such long suspensions the tube must be very 

 straight, and the arrangement for raising and lowering the 

 suspended parts must be very good. After trying several 

 arrangements for this, I adopted a pointed piston working 

 into the tube and passing through a stuffing-box. The coils 

 were of course anything but suited, as far as shape goes, for 

 their present arrangement ; however, they did what was 

 requisite, though I have no doubt that coils might be made 

 to increase the sensitiveness tenfold. 



The mirror was in this case suspended midway between the 

 magnets, and, for want of a better, was so thin that it got a 

 little pulled out of shape by the paint which was used to fasten 

 it to the aluminium wire. This fortunately turned out to be 

 an advantage in some respects, for the vertical wire and the 

 paint on the back of the mirror, by a happy accident, made 

 the figure of the mirror practically that of a portion of a 

 cylinder with a vertical axis ; consequently, using a very good 

 lens of 40 inches' focus, a good image of the light-spot was 

 obtained at a distance of three metres. It became evident at 

 once that the two real difficulties in securing sensitiveness 

 lay in preventing air-currents and in adjusting the controlling 

 magnet. 



The first was finally attained by making the instrument 

 practically air-tight ; and, by means of a diaphragm, stopping 

 down the beam of light to very nearly the size of the mirror 

 — in this case of about 1 centimetre diameter. During the 

 measurement of the resistance of an impure sample of sulphur 

 this protection against air-currents was found to be insufficient, 

 and the galvanometer was further protected by enclosing it 

 in a cardboard box. If it ever becomes necessary to make an 

 instrument to be sensitive to, say, 10" 13 amperes, I shall have 

 the support for the controlling magnet absolutely independent 

 of the galvanometer-case, so that any vibrations set up in 

 adjusting the magnet shall be transmitted only through heavy 

 masonry. In the galvanometer now being described the con- 

 trolling magnet could be raised by a nut and screw combina- 

 tion, itself sliding on a brass tube attached to the case of the 

 instrument. The glass tube containing the fibre was clamped 

 at its upper end to a very heavy stand of brass and lead, and 

 this practically sufficed when the sensitiveness got to be of the 

 order of one division to 10 -11 amperes. 



