If you pull the lever handle around one turn, its outer end has described a circle. The circumference of this circle is equal to  2x. (Remember that  n equals 3.14, or 22/7). That is the distance you must apply the effort of the lever arm. At  the  same  time,  the  screw  has  made  one revolution, raising its height to equal its pitch  (y). You might say that one full thread has come up out of the base. At any rate, the load has risen a distance  p. Remember that the theoretical mechanical advan- tage (T.M.A.) is equal to the distance through which you apply the effort or pull, divided by the distance and resistance the load is moved. Assuming a 2-foot, or 24-inch, length for the lever arm and a 1/4-inch pitch for the thread, you can find the theoretical mechanical advantage  by  the  formula 27tr T.M.A.   =   — P in that r   = length of handle = 24 inches p =  pitch,   or   distance   between   corresponding points on successive threads = 1/4 inch. Substituting, A 50-pound pull on the handle would result in a theoretical lift of 50 x 602 or about 30,000 pounds—15 tons for 50 pounds. However, jacks have considerable friction loss. The threads  are  cut  so  that  the  force  used  to  overcome friction is greater than the force used to do useful work. If the threads were not cut this way and no friction were present, the weight of the load would cause the jack to spin right back down to the bottom as soon as you released the handle. THE MICROMETER In using the jack you exerted your effort through a distance of 2nr, or 150 inches, to raise the screw 1/4 inch. It takes a lot of circular motion to get a small amount of straight line motion from the head of the jack. You  will  use  this  point  to  your  advantage  in  the micrometer; it’s a useful device for making accurate small  measurements—measurements  of  a  few thousandths of an inch. In  figure  5-3,  you  see  a  cutaway  view  of  a micrometer. The thimble turns freely on the sleeve, Figure  5-3.-A  micrometer. Figure  5-4.—Taking  turns. rigidly attached to the micrometer frame. The spindle attaches to the thimble and is fitted with screw threads that move the spindle and thimble to the right or left in the sleeve when you rotate the thimble. These screw threads are cut 40 threads to the inch. Hence, one turn of the thimble moves the spindle and thimble 1/40 of inch. This represents one of the smallest divisions on the micrometer. Four of these small divisions make 4/40 of an inch, or 1/10 inch. Thus, the distance from 0 to 1 or 1 to 2 on the sleeve represents 1/10, or 0.1, inch. To allow even finer measurements, the thimble is divided into 25 equal parts. It is laid out by graduation marks around its rim, as shown in figure 5-4. If you turn the thimble through 25 of these equal parts, you have made  one  complete  revolution  of  the  screw.  This represents a lengthwise movement of 1/40 of an inch. If you turn the thimble one of these units on its scale, you have moved the spindle a distance of 1/25 of 1/40 inch, or 1/1000 of an inch—0.001 inch. The micrometer in figure 5-4 reads 0.503 inch, that is the true diameter of the half-inch drill-bit shank measured. This tells you that the diameter of this bit is 0.003 inch greater than its nominal diameter of 1/2 inch—0.5000  inch. 5-2