Leaders. 137 



close contact or not it would require exactly the same 

 strain to again extend the spring to the same length. 

 It is also equally clear that this strain could be reap- 

 plied, and, at the same time, be measured by a spring- 

 balance. Therefore the point to be automatically reg- 

 istered was simply this : how far had the spring been 

 pulled out that is, how much had it been elongated ? 

 Now suppose we fasten across the last coil at the reel- 

 end of the spring a piece of wood or brass with a small 

 hole through it in line with the axis of the spring. Now 

 let us thread a string which we will call the " meas- 

 uring-cord " through this hole, and leading it length- 

 wise inside the spring, fasten it firmly to the fish-end of 

 the spring. The cord must so fit the hole in the brass 

 or wood, that while it may easily be pulled through it 

 in either direction, it will stay where it is left. 



The result will be that, when the spring is stretched 

 out, the measuring-cord is pulled through the hole in the 

 wood or brass at the reel-end of the spring. When the 

 strain is removed and the spring returns to its original 

 length, the measuring-cord will not repass the hole, but 

 fold up inside the spring. Now if we mark the meas- 

 uring-cord just outside the hole by nipping it there in 

 a split-stick, or tying a different-colored string tight 

 around it, we can then pull out as much of the measur- 

 ing-cord as is inside the spring, apply a spring-balance 

 to the fish-end of the spring and pull the spring out till 

 the measuring-cord is drawn through its hole to the 

 same point as before. The spring will then have been 

 elongated to the same extent as before, the strain re- 

 quired to do this can be read from the spring-balance, 

 and we know just how much the fisn really pulled at 

 its maximum effort. 



