Ican't find this answer anywhere, so i'll ask here: what item did they base the si unit of mass (grams) off of? (in other words, to what object did they say "hey, lets make this equal 1 gram and then go from there"

Currents in dc transmission lines can be 100 a or higher. some people are concerned that the electromagnetic fields from such lines near their homes could pose health dangers. for a line that has current 150 a and a height of 8.0 m above the ground, what magnetic field does the line produce at ground level? express your answer in teslas and as a percentage of the earth’s magnetic field, which is 0.50 g. is this value cause for worry?