N.H. Rev. Stat. § 479:28
From and after January 1, 1960, all undischarged mortgages of real estate which shall have been on record for a period of more than 50 years and which attain such age subsequent to January 1, 1960, shall be void; provided, however, that a mortgagee or assignee of such a mortgage may continue the same in its full legal effect and priority for an additional period of 50 years from date of such action by recording at any time within such 50-year period an affidavit signed by the mortgagor and the mortgagee or assignee stating that the mortgage is not discharged but is still in full force and effect. All such mortgages which shall have been on record for a period of more than 50 years on or before January 1, 1960, shall become void on January 1, 1960, unless prior to that date the mortgagee or the assignee of such a mortgage shall have recorded an affidavit signed by the mortgagor and the mortgagee or assignee stating that the mortgage is not discharged but is still in full force and effect, and upon the recording of such an affidavit such mortgage shall retain its full legal effect and priority for an additional period of 50 years from the date of such recording.
RSA 479:28
1957, 256:1, eff. Sept. 27, 1957.