262 THE POPULAR SCIENCE MONTHLY. 



suspended on it. I hold the bar, and turn it half round ; it swings 

 backward and forward for a few times, but it quickly comes back to 

 its original position. However much twist, however much torsion, 

 may be put on this, it always returns ultimately to the same position. 

 I have twisted glass fibres round and kept them in a permanent state 

 of twist more than a hundred complete revolutions, and they always 

 came back accurately to zero. The principle of an instrument that I 

 shall describe farther on depends entirely on this property of glass. 



Instead of using silk to suspend the torsion-beam with, I employ a 

 fibre of glass, drawn out very fine before the blow-pipe. A thread of 

 glass of less than the thousandth of an inch in thickness is wonder- 

 fully strong, of great stiffness,- and of perfect elasticity, so that, how- 

 ever much it is twisted round short of the breaking-point, it uutwists 

 itself perfectly when liberated. The advantage of using glass fibres 

 for suspending my beam is, therefore, that it always returns accu- 

 rately to zero after having tried an experiment, while I can get any 

 desired amount of sensitiveness by drawing out the glass fibre suffi- 

 ciently fine. 



Here, then, is the torsion apparatus sealed on to a Sprengel pump. 

 You will easily understand the construction by reference to the dia- 

 gram (Fig. 4). It consists of an horizontal beam suspended by a glass 

 fibre, and having disks of pith at each end coated with lampblack. 

 The whole is inclosed in a glass case, made of tubes blown together, 

 and by. means of the pump the air is entirely removed. In the centre 

 of the horizontal beam is a silvered mirror, and a ray from the electric 

 light is reflected from it on to a scale in front, where it is visible as a 

 small circular spot of light. It is evident that an angular movement 

 of the torsion-beam will cause the spot of light to move to the right 

 or to the left along the scale. I will first show you the wonderful 

 sensitiveness of the apparatus. I simply place my finger near the 

 pith-disk at one end, and the warmth is quite sufficient to drive the 

 spot of light several inches along the scale. It has now returned to 

 zero, and I place a candle near it. The spot of light flies off the scale. 

 I now bring the candle near it alternately from one side to the other, 

 and you see how perfectly it obeys the force of the candle. I think 

 the movement is almost better seen without the screen than with it. 

 The fog, which has been so great a detriment to every one else, is 

 rather in my favor, for it shows the luminous index like a solid bar of 

 light swaying to and fro across the room. The warmth of my finger, 

 or the radiation from a candle, is therefore seen to drive the pith- 

 disk away. Here is a lump of ice, and on bringing it near one of the 

 disks the luminous index promptly shows a movement of apparent 

 attraction. 



With this apparatus I have tried many experiments, and among 

 others I endeavored to answer the question, " Is it light, or is it heat, 

 that produces the movement?" for that is a question that is asked 



