chapter
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M

The input force is called the effort and the output force is called the load. If there is no friction:

energy transferred from effort = energy transferred to load

effort x effort distance = load x load distance

effort distance _ effort arm length load distance ~ load arm length

effort arm length load = effort x —:— --------- ;— ~n~load arm length

If friction is present, part of the effort is used against friction and the load is correspondingly smaller. It is usual to define three quantities:

load mechanical advantage (m.a.) = eff-o rt

e ffort distance velocity rat,o (v.r.) = - oa^ dj ^ n“

mechanical advantage eff,oency - ve iocityratio X l00/o

The “velocity ratio” is always a precise function of the way a machine is built. It is cal­ culated from the lengths of the levers or the number and diameter of the pulleys. Either the load is fixed and the effort is measured or the effort is fixed and the load measured. These values give the mechanical advantage. The efficiency can then be calculated. (See efficiency (machines).)

magnet: a block of permanently magnetized ferromagnetic material, usually with opposite poles at the far ends. (Se e ferromagnetism.)

magnetic effect of current: the power of electric current to create a magnetic field in its immediate vicinity. The diagram shows three cases: a straight wire, a

ring and a solenoid. The right-hand grip rule gives the direction of the magnetic field. For a straight wire, the thumb points along the current direction and the fingers fol­ low the direction of the circular magnetic field lines. For the solenoid, the fingers follow the current direction and the thumb points along the solenoid toward the north-seeking pole. The direction of the magnetic field near the ring can be worked out from either of these applications.