We derive the electron temperature gradient in the Galactic disk using a sample of H ii regions that spans Galactocentric distances 0–17 kpc .
The electron temperature was calculated using high precision radio recombination line and continuum observations for more than 100 H ii regions .
Nebular Galactocentric distances were calculated in a consistent manner using the radial velocities measured by our radio recombination line survey .
The large number of nebulae widely distributed over the Galactic disk together with the uniformity of our data provide a secure estimate of the present electron temperature gradient in the Milky Way .
Because metals are the main coolants in the photoionized gas , the electron temperature along the Galactic disk should be directly related to the distribution of heavy elements in the Milky Way .
Our best estimate of the electron temperature gradient is derived from a sample of 76 sources for which we have the highest quality data .
The present gradient in electron temperature has a minimum at the Galactic Center and rises at a rate of 287 \pm 46 { { K kpc ^ { -1 } } } .
There are no significant variations in the value of the gradient as a function of Galactocentric radius or azimuth .
The scatter we find in the H ii region electron temperatures at a given Galactocentric radius is not due to observational error , but rather to intrinsic fluctuations in these temperatures which are almost certainly due to fluctuations in the nebular heavy element abundances .
Comparing the H ii region gradient with the much steeper gradient found for planetary nebulae suggests that the electron temperature gradient evolves with time , becoming flatter as a consequence of the chemical evolution of the Milky Way ’ s disk .