  Compacted platinum powders exhibit a sharp onset of diamagnetic screening at
$T \simeq 1.9$ mK in zero magnetic field in all samples investigated. This
sharp onset is interpreted in terms of the intragranular transition into the
superconducting state. At lower temperatures, the magnetic ac susceptibility
strongly depends on the ac field amplitude and reflects the small intergranular
critical current density $j_{c}$. This critical current density shows a strong
dependence on the packing fraction f of the granular samples. Surprisingly,
$j_{c}$ increases significantly with decreasing f ($j_{c}(B=0, T=0) \simeq
0.07$ A/cm$^{2}$ for f = 0.67 and $j_{c}(B=0, T=0) \simeq 0.8$ A/cm$^{2}$ for f
= 0.50). The temperature dependence of $j_{c}$ shows strong positive curvature
over a wide temperature range for both samples. The phase diagrams of inter-
and intragranular superconductivity for different samples indicate that the
granular structure might play the key role for an understanding of the origin
of superconductivity in the platinum compacts.
