110 EXTENSOMETE11S. 



Some excellent machines of this kind are made by one 

 or two American firms, who have especially devoted them- 

 selves to, and seem to excel in, this kind of work. 



60. Bar Calipers. For the purpose of taking 1 compara- 

 tively rough measurements, a pair of short bar calipers will 

 be found useful. These are similar to the vernier calipers 

 which have been described, with the difference that there is 

 no vernier attachment and no screw adjustment, so that 

 measurements can only be made to hundredths instead of 

 thousandths of an inch. They are quickly manipulated, 

 and will be found very useful for many purposes. 



EXTENSOMETERS. 



61. It is necessary in almost all kinds of tests to take 

 measurements of the elastic deformations or strains pro 

 duced by the stresses applied. In most specimens of ordinary 

 size these strains are extremely small, and consequently the 

 instruments used for measuring them require to be specially 

 adapted for exhibiting minute changes in the dimensions 

 of the specimen. In the simplest and most frequently 

 performed tests, namely those in tension and compression, 

 it is alterations of length which have to be recorded, and 

 the instruments used for this purpose are called extenso- 

 meters. Some of the more important of these will now be 

 described. There are many different extensometers in use, 

 almost every experimenter having his own particular 

 design. In order to understand what are the necessary 

 requirements to be fulfilled by an extensometer, consider a 

 tension specimen having eleven inches clear length between 

 the holding shackles. We will suppose loads to be applied 

 which do not exceed the elastic limit. In the case of the 

 metals a bar is stretched to somewhere about one thousandth 

 of its own length before the elastic limit is reached. If the 

 length upon which measurements are taken is 10 in., as 

 would be convenient in the present case, the whole elastic 

 stretch of the bar will be one thousandth of ten inches, 

 that is to say one-hundreth of an inch, so that the actual 

 alterations of length due to comparatively small increments 

 of load will be small fractions of one-hundredth of an inch. 

 It is found that it is necessary in most cases to be able to 

 read extensions to thousandths and ten thousandths of an 

 inch. These are very small quantities, which cannot be 

 read by ordinary means, and some method must be employed 

 by which they can be rendered visible to the eye. This is 



