We are now able to explain the principle of the beam balance, the construction which was discussed in chapter I, The unknown mass m1, is placed in the left-ha pan and standard masses, rn-, are placed in the right-hand pan to obtain equilibrium (Fig. 6.1 0). The balance has been constructed so that the distances, d, of the pans from centre beam bearing are equal. Thus equating moments

From this we see that the balance measures mass, not weight, and it is quite independent of the value of g. Furthermore, it will correctly measure mass anywhere so long the masses concerned possess weight. Should we want to know the weight of the body we could calculate it by multiply mg the mass by the local value of g. What has been said above about the beam balance should be compared with our discussion on the spring balance in chapter 4.