3789520

Information

• Patent Grant
• 3789520

• References
• Source

• Patent Number

  3,789,520

• Date Filed

  Not available

• Date Issued

  Tuesday, February 5, 1974
  50 years ago

• CPC

  • G09B3/085 - with invisibly printed correct answer; the answer being rendered visible by special optical means

• US Classifications

  • 434 - Education and demonstration